Dollar bills seems a little bit cheesy. Twenties would have made a big enough pile.
Nah, the whole point was to have such an overwhelming physical representation of the reward. A check for $15K is sliced by taxes and allocated to something mentally with little physical stimulus. It's only $750 twenties which would be about the size of a hardback book. In ones, it's literally a briefcase full of money, so even though the amount is the same, you're like a guy in a movie, not just someone who's going to pay off a credit card.
It just seems to me that the spectacle of it would be undermined by the annoying amount of time spent picking stray bills off the floor.