It's been reported that the Broncos wanted an "early third round" pick for Lelie. Assuming that this is indeed what the Broncos wanted, here are some examples for the various scenarios:
An early 3rd rounder is worth ~240 points.
Swapping first rounders only: This would happen if, for example, the Broncos lose the AFCCG (draft pick ~620 points) and the Skins just miss the playoffs (around the 20th pick, worth ~850 points).
Broncos get the Skins 3rd rounder: The Skins would have to be pretty bad for this scenario. Think 4-5 wins. Probably not gonna happen.
Broncos get 3rd rounder plus swap first round picks: This would happen if, for example, the Broncos lose the AFCCG (~620) and the Skins make the playoffs but lose in the divisional round (3rd ~145, 1st ~720 so swapping 1st has a value of about 100)
Broncos get 3rd and 2008 4th: I assume this is in play if the Skins finish better than the Skins.
Broncos get 2008 4th and swap 1st rounders: This would likely happen if the two teams finished right next to each other or a couple of spots apart, as a 2008 fourth rounder is worth about 35 points, and the late 1st rounders differ in value by 20 points.
Clear as mud?