NFL Value Chart
I will concede that if Adrian Peterson is the overall number one selection that trading up would be very prohibitive, assuming arguendo, that the Cowboys have any success at all this year. According to one value chart, the point value of the top 5 picks would be as follows:
#1: 3000 pts.
#2: 2600 pts.
#3: 2400 pts.
#4: 2200 pts.
#5: 1900 pts.
As you can see, if Peterson drops in the draft a few spots, the Cowboys' ability to trade up goes up dramatically.
If the Cowboys had decent success this year, but didn't reach the Super Bowl (a possibility), their first round pick this year would likely be worth approximately 700 points (#27). It would likely cost at least two first round picks to get into the top 5. It is more likely that the cost would be greater than two first round picks, and would require the Cowboys to part with a player in addition to the two first rounders. I am guessing it would take two first rounders, plus a healthy semi productive Julius Jones to even get close if Peterson was in the top 3 picks. Still, if Peterson lights it up this year, I would make the trade in a heartbeat. If he doesn't light it up, he won't be a top 5 pick as a running back.