Cash prizes are less interesting for this kind of thing. (various studies)
I've read the same, but I don't believe it. For example people will say that fame is more important than money, but I would argue fame leads to more work, or more free stuff which in turn leads to money or more money than a set cash incentive. Or the case where people do things for points, but when those points are redeemable for physical items or discounts again this leads right back to some sort of cash savings. I think it always go back to some sort of monetary incentive.