Quick reference: You may download the calculator here.
Since the dawn of trading draft picks, everyone has tried to determine the "fair value" for a particular selection. Calculators abound, including our very own David Dodds' Calculator found here. Unfortunately for leagues that span multiple years, these calculators have little or no value. So how can you adequately calculate what a draft pick is worth in these formats?
Dynasty vs. redraft leagues
First, you have to understand the difference between dynasty leagues and redraft leagues. In a redraft league, you are essentially posing the question of "how will Player X help me over the next few months?" In dynasty leagues, that question is in terms of years rather than months, as you keep players multiple seasons (perhaps their entire career). That is the essence of the problem  how do you quantify a multiyear value?
To attempt a solution to this problem, I sought out two different sources of information. The first was the only multiyear valuation chart that I know of that exists  the NFL Draft Pick Chart, which gives a numerical value to every pick in the NFL Draft. This chart is the most useful one to dynasty league players, since the NFL is in essence a dynasty league. The value of each pick factors in that the selected player is supposed to play for their club for years.
The second resource I needed was to answer the natural followup question, "how many years?" With the help of fellow Footballguys, I found a study performed at Dartmouth University regarding the average career length of an NFL player, both in aggregate and also broken down by position. Now, many would question the validity of using that data, as some would say that there is no way to predict the length of a selected player's career or that a first rounder may last longer than a fourth round selection, but that is precisely why I selected that chart. The chart is the length of an average NFL career regardless of the round they were drafted, or even if they were an undrafted player. The data may vary by round (and that would be an interesting study), but for the purposes of pick calculations I have assumed that all NFL rookies have the same expected length of career associated with their position.
Position 
Years

Quarterback 
6.96

Running Back 
4.35

Wide Receiver 
4.54

Tight End 
4.98

Defensive Lineman 
5.05

Defensive Back 
5.51

Linebacker 
5.50

Place Kicker 
8.33

Team Defense 
n/a

All Positions 
5.33

Table 1  Average Career Length of NFL Players
the dynasty factor
There are a few different variables that enter in to calculation of a pick value for a multiyear league. First is career length, but what about the league itself? How many teams are there? How big or small is the roster? How deep is the bench? How many starters? What matters, and by how much?
What I have done is rather complicated, but I have rolled all these numerous factors into a secret formula that produces a single number that I have labeled the "Dynasty Factor". The Dynasty Factor is the numerical value for your entire league, and it represents how valuable your draft picks are for your league. It can be used to compare different dynasty leagues with different rosters and starters, and gives an independent value to each league.
Rather than have everyone see the mathematical analysis and details of the "Secret Sauce", I will lay out some of the proportional relationships for the Dynasty Factor that you will see when you first start to use it. I will tell you that the following factors contribute to the Dynasty Factor in some way:
 Roster Size
 Number of Teams in the League
 Number of Starters
 Position(s) of Starters
The influence of each of these factors is complex, but I will attempt to give you a more intuitive feel for the tool as you start to take it for a test drive.
Roster Size  The pick values are indirectly proportional to your roster size. That is, as your roster size increases, the values decrease. To say it one more way, they are inversely related.
Why would that be? Well, if you have a deep bench, you can afford to put more rookies on your roster and wait for them to develop, so there is less pressure on you getting your picks right. You also have more room for veterans on your squad, so both factors reduce the pressure on getting the pick right (and right away), and thus the pick values go down accordingly. On the other hand, the Dynasty Factor goes up as your bench gets smaller. This makes sense  you have to "hit" on your rookies more often and they have to develop faster, else they will get cut in favor of other players. You don't have room to develop players over a period of years with a short bench.
Number of Teams in the League  As the number of teams increases, the value of the picks increase. Again, this passes the sanity check in that you have fewer draft picks and more teams are fighting for talent. Additionally, more players are rostered in the league and thus fewer talented players are available in free agency.
Number of Starters  As the number of starters increases (relative to your roster size), the bench gets shorter and the pressure on getting a good value in the draft goes up. This makes sense, as the rookie picks matter more when you start more players. Therefore, the value also goes up, and therefore Dynasty Factor is directly related to starters.
Position of Starters  This relationship is complicated, but suffice it to say that the more players that you have with shorter careers, the higher the Dynasty Factor for your league. This indicates the "rollover" of your roster, or how fast you have to churn talent through your team. If you are losing players every three or four years, you need a constant stream of young talent from the draft, increasing the value of your picks.
Note  The Dynasty Factor is NOT directly proportional to the ration of the number of starters divided my your team roster size. Why? Well, if you have 8 starters and 24 roster spots, finding starters is easier than if you have 16 starters and a bench of 48.
you down with idp? (Do you know me?)
Can the Dynasty Factor be used with IDP leagues? Absolutely. In fact, it shows something that many who play IDP probably already knew, but could not quantify. The addition of individual defensive players minimizes the impact of the offensive players, and also the typical influence of rookies.
Why would that be? Well, for one thing, many IDP leagues have roughly the same number of players on both sides of the ball. For example, that would mean eight IDPs for eight offensive players, minimizing any standout offensive player. In a regular league, Calvin Johnson or Peyton Manning would be one out of only eight starters, but in the IDP example he is just one of out of 16, so his individual influence on the total team score is less in an IDP format than in a league that uses a team defense.
Examples
Let's go over the Dynasty Draft Pick Calculator and a few examples.
I have set up two typical leagues, a regular league with eight starters and a team defense, and another with 16 players, half IDP. Here are the respective Dynasty Factors (See Tables 2 and 3).
Dynasty  Typical
League Starters 

QB

1

RB

2

WR

3

TE

1

PK

1

Team D

1

DL

0

LB

0

DB

0

Total

9

Roster Size

27

Dynasty Factor

3.466

Dynasty  Typical IDP
League Starters 

QB

1

RB

2

WR

3

TE

1

PK

1

Team D

0

DL

2

LB

3

DB

3

Total

16

Roster Size

48

Dynasty Factor

2.663

Table 3  Dynasty Typical IDP League Starters and Roster Size
As expected, the IDP league has a smaller Dynasty Factor.
We still need frames of reference to show how this really works. Let's compare the values of the first 100 Draft Picks for a Redraft League, the NFL Pick Chart, and the two leagues above (see Table 4).
Pick

Footballguys
Redraft 
Dynasty
Typical 
Dynasty
Typical IDP 
NFL
Chart 
NFL Chart
Normalized 
1

1889

1889.00

1889.00

3000

1889.00

2

1823

1669.93

1718.31

2600

1637.13

3

1759

1475.38

1562.32

2200

1385.27

4

1699

1308.16

1424.40

1800

1133.40

5

1642

1162.24

1300.66

1700

1070.43

6

1587

1032.79

1187.85

1600

1007.47

7

1535

920.16

1087.01

1500

944.50

8

1485

820.38

995.25

1400

881.53

9

1438

733.84

913.56

1350

850.05

10

1393

657.27

839.40

1300

818.57

11

1351

591.10

773.68

1250

787.08

12

1310

531.22

712.72

1200

755.60

13

1272

479.69

658.98

1150

724.12

14

1235

433.04

609.17

1100

692.63

15

1200

391.97

564.27

1050

661.15

16

1167

355.86

523.88

1000

629.67

17

1135

323.17

486.50

950

598.18

18

1105

294.51

453.00

900

566.70

19

1076

268.58

422.03

875

550.96

20

1049

245.93

394.41

850

535.22

21

1023

225.45

368.91

800

503.73

22

998

206.92

345.39

780

491.14

23

975

190.85

324.60

760

478.55

24

952

175.70

304.60

740

465.95

25

931

162.63

287.04

720

453.36

30

836

111.99

215.51

620

390.39

35

759

80.12

166.61

550

346.32

40

694

58.74

131.27

500

314.83

45

638

43.89

104.92

450

283.35

50

586

32.68

83.66

400

251.87

55

538

24.31

66.63

350

220.38

60

491

17.71

52.24

300

188.90

65

446

12.69

40.44

265

166.86

70

402

8.85

30.67

240

151.12

75

358

5.92

22.52

245

154.27

80

316

3.84

16.15

190

119.64

85

276

2.40

11.27

165

103.90

90

239

1.46

7.68

140

88.15

95

204

0.84

5.04

120

75.56

100

173

0.48

3.25

100

62.97

Table 4  Comparison of Pick Values
Lastly, they say a picture is worth a thousand words, and I agree based on the next chart. This is the graph of pick values for all four leagues, with the NFL pick chart normalized to the FBG chart (setting the #1 pick to the same value).
Figure 1  Rookie Draft Pick Values
What can we take away from Figure 1? Well, The Normalized NFL Draft Chart (green) decreases in value far faster than the Footballguys Redraft Calculator (black and yellow), as we expected. The other two typical dynasty leagues are far steeper overall in the pick value. Is that expected? I would say without reservation that this is accurate, since the NFL chart includes nonskill position players such as offensive linemen and kickers, which last far longer. The skill position players (and their values) should decline rapidly, which is exactly what the graph shows. Looks like we have something here.
let's get started
Okay, enough already. What do I need to do to try this out?
First, here is the main screen for the Dynasty Draft Pick Calculator.
Figure 2  The Dynasty Draft Pick Calculator  Rookie Values
You may download the calculator here. The screen above appears complicated, but it isn't as daunting as you may first think.
I have colorcoded what you are expected to enter, starting with the orange, blue, and pink sections. In order:

Orange  Enter the number of teams in your league and the starting positions, along with your total roster size.

Blue  Enter the picks that you are considering trading away (the number of the pick in integer form, namely Pick 14, not 2.02 in a 12team league).

Pink  Enter the picks that you are looking to obtain in a trade (again in integer form)
That's it. The math works behind the scenes, as a Dynasty Factor is calculated for the league and the relative pick values are given to you as a result. You can enter up to eight picks on either side of the transaction, and the league size will adjust the selection to show it in the "round . pick" format (e.g. Pick 14 is 2.02).
The key sections to watch are your "% Diff." and the "Pick to Make Even" at the bottom of the screen. The color of the "% Difference" will change to reflect the calculator's opinion of the trade  red is a bad trade, green is a good one, and yellow indicates a fair trade (values are close enough).
A wrinkle I added is the "Pick to Make Even". If you enter the pick suggested here in the calculator on the suggested side (blue or pink), the suggested trade will result in close to equal value. This helps in trying to determine a fair trade.
future work
The Dynasty Factor is a new metric, and I hope it is revolutionary. I do know that it is evolutionary, and it will continue to be tweaked as I develop this calculation.
As far as I know, the Dynasty Calculator for Rookie Picks is a first of its kind. There may be some bugs to work out, but overall I am quite happy with how it turned out. Obviously further work can be done  there are other league variations (keepers, startup dynasty, veterans and rookies, etc.) to work on, as well as incorporating a few other wrinkles such as flex positions and other factors such as players already associated with teams in the league.
Reference:
http://www.dartmouth.edu/~chance/chance_news/recent_news/chance_news_11.02.html#item3