2015 Contract info...
http://www.spotrac.com/nfl/draft/
He needs to decide how likely it is he can move up a round by staying. If he can move into the 1st or 2nd round by staying, he should stay. Note the huge drops in guaranteed money for last the picks of rounds 1 and 2 and the first picks in rounds 2 and 3. The money differential between pick 32 and pick 65 is massive.
I agree that there is a pretty big difference in rounds 1, 2, and 3.
However, if Collins has a good first 3 years, he will be able to sign a big 2nd contract and make huge money. He would also be getting that 2nd contract a year earlier than he would if he stayed at NU for his senior year. I think that's the point people are missing with guys coming out of college. Basically, Collins would be making a bet on himself to progress and get better. I think a guy progresses more with a year in the NFL, where his only job is football, instead of doing it part-time in college.