I think that it's a mixture of a few things. Jews DID come from very modest backgrounds, and they were always very good with money and dealt with money lending so that's where that comes from. Also the idea of the cheap Jew comes from what AussieJTF said. When jealous gentiles see rich Jews, they have to turn that into a negative thing. They can't admit that the Jew is smart, inventive, creative or hard-working, because to do that would be admitting your own inferiority, so how do you turn being rich into a negative thing? Just say that the only reason the Jew got rich is because he pinches every penny and doesn't spend a dime of it, then all the anti-Semites can say "If I was such a greedy and selfish pinny-pinching miser, I would be as rich as he is!"