i think a lot of people in this thread are looking at this all wrong.
1) UNC is paying this money and getting in the "Bill Belichick business" because they see it as a ticket into the Big 10 or SEC once the Big 10/SEC splinter off and do their own thing. It makes them much more attractive to both conferences. Having Bill as their head coach for a few years puts them in a new light, changes the perception around them, and gives them some cache.
2) From Belichick's perspective, he is only taking this job to get back into the NFL. If an NFL team called him tomorrow and made him an offer, he would be taking it. The red flag with him is that he is old and cannot connect to the "young" and "modern" athlete. If he can go to UNC for a year or two, have mild success, fake crack a smile and look like he is having fun with the 19 year olds, win 8 games or so each year, then an NFL team may come calling then with a job offer where he can break Shula's record. 75 is the new 55.
There is a "LONG PLAY" here both for UNC and Bill, and they are using each other.