I think its a good idea - anything to motivate the kids in learning and spending their time in useful ways in which they can also benefit from is awesome!
A potential problem I foresee is that (rival) classmates down-voting each other or spamming to get their own posts rating higher - or down-voting, flagging each other as bullying.
Every good idea has its snags and I'm sure there are creative ways around these problems.
Well the amount of money it makes in the end doesn't really matter for their class grade. Just how well they do on the assignment. Maybe I'll implement it in my programming class.