NFL GM's use some variation on the value chart developed by Mike McCoy in Dallas in the 90's. It's one of those things like the passer rating formula...nobody knows exactly why it works, but there's a broad consensus that it does.
While each team has some variation of their own, the charts you'll run across are remarkably similar. While the charts function as rules of thumb or a baseline for considering a trade, and while some conjectures must be made about where the trading partner will finish the following year if future picks are involved, if one is to spit ball a trade the charts are the best basis for doing so.
Here's a typical chart:
http://www.draftcountdown.com/features/Value-Chart.php
Assuming the Packers sit in the 30 spot this year and next, as an example, then a trade up for the #5 pick in 2014 (1700 points) would cost something on the order of the two first rounders, a second rounder and a third rounder (1648 points). An entire draft from the 30 spot would be 1124 points.