Well Vick's deal last year could earn him up to $63 million. That is why teams are trying to trade out of the first few picks all the time. The top 5 picks are huge cap busters, especially if the player isn't performing at a high level by their third season. It is hard to watch an $8 million cap hit sit on your bench and learn for a year or two, when in fact $8 mil can fill so many other needs.
NBA has the best system for rookie contracts, the NFL should look into it. Teams are breaking themselves financially on these high picks, too much cap room is used up for a rookie.
Top 5 picks could be as much as 6-12% of your cap space. Dangerous game to be playing with such young, unproven players.