You're looking at this all wrong. The money difference between a rookie deal in the top 5 and rookie deal in the 10-15 range is only a few million a year. He's also going to get endorsements and a shoe deal, which equals more money.
On top of all of that, he's starting his clock to get his 2nd contract and that's the one where he'll really get paid. Then he'll get another big deal for his 3rd contract. Coming back to IU isn't guaranteed to raise his draft stock either. We can all assume that it will, but that's a big assumption. Had he been able to go pro straight out of HS then he'd have been a top 5 pick LAST year.
http://www.cbafaq.com/scale17.htm