While I understand Ball wants to be drafted higher than the third round (read that was where he was projected), the fact is that good running backs are a dime-a-dozen in the NFL. Why would teams waste money and a pick on a high round when teams can win just as easily without a top RB. Look at the Packers, Saints, and Patriots. Great QBs meh running games although the addition of Darren Sproles has made the Saints great.
...and beyond that, I'm not going to do the calculation myself, but even if he improves his draft stock a full round, I can't see this being a good financial decision. I wouldn't expect that second vs. third round money would make up for the 1 year of paychecks he's missing (especially given the average career length of NFL rbs.)
That said, I'm sure he's thought about it plenty, and made his call. Good on him for staying in school and sticking with his folks and rumored GF. Here's to hoping he has a horrible season and ruins his NFL prospects.