The one thing that is consistent about general managers is that they want to be able to put together a team that suits their own personal visions. They tend to trust their own judgment (or, at least, the judgment of their staff). As a result, just like kids on the playground, when it comes to picking players they want to go first. A GM giving up a spot in line is worth something usually worth an extra turn, or at least a slightly better turn, the next time around. Figuring out how each turn in line compares to each other turn is always tricky, but over time a general sense of how much each pick was worth was agreed upon.
For a long time, the “Jimmy Johnson Chart” held sway, with the overwhelming majority of trades coming within 5% either way of the values listed on that chart. However, with the new salary cap and other changes over time, a new chart seemed to be developed. Rich Hill so far seems to have done the best job of figuring out the values on this chart, and between the two it’s pretty easy to figure out how much each pick should be worth. In simple terms, the #1 overall pick in the draft is the most valuable option.
Depending on which chart is being used, picking #20 (where the Chicago Bears are slated to go) is worth only 27-28% as much in total “trade value” as picking #1. Likewise, picking #52 is worth 11-13% as much as #1. By the time the last few rounds roll around, pick #204 ends up being worth only 0.3% or 0.4% of the value of picking #1. This almost always means that by moving later in the draft, a team gets more choices, whereas a team has to give up turns (sometimes a lot of them) to bump in line.
Of course, it’s not always that simple.
If a GM doesn’t have enough lunch money to outbid a rival this year, he can always follow the J. Wellington Wimpy model of spending and offer future picks. Of course, just like those kids on the playground, GMs are really impatient. Waiting a whole year for their presents is just too hard, and so a future pick is almost always valued at the same rate as a pick in the middle of the next round. This would mean that a future first-rounder is valued like a middle second-rounder. In this way, GMs bet on themselves to be better than average, while the GMs that they trade with hope their trade partners falter and up worse than average.
Another issue to keep in mind is which picks are being traded. Imagine that I am the GM for the team about to draft next and there are only four players I really like who are left. If another team wants me to trade down ten spots, I might not risk it even if they offer a really good price. There’s too much chance I’ll miss out on all four players. On the other hand, of there are twenty players I really like and someone is offering me two chances to pick (say, 10 spots and 15 spots from now), then I might trade in a hurry.
Sometimes actual players, not just chances to pick players are offered. In these cases, it’s really hard to say what the player is worth. It’s possible to do “comps” on similar players, but it’s never apples to apples. This is actually good news, because involving a player can sometimes be a win-win. Each side can convince themselves that they got the better deal, and salary commitments can be handed off. Ultimately, though, players are usually valued less by GMs than by their fans, with even starter-caliber players being traded for picks that might not even make the roster in a year.
Obviously, there are nuances. However, these are the basic factors that influence trades in the draft.