I have had the same concerns, but to be fair there are a few "outs." In one episode he takes a stack of cash, that if composed of $100 bills, would be over $100k easy. Or at least $20k if it's 20's. Fiona makes money herself and we're supposed to believe that Sam does as well, at least by mooching off the ladies. Though he also got a pile of cash the same size as Michael's in the same episode (#6).