In the scene where they had performed in the club for tips, you see Ben counting a large number of bills. After he punches that guy who was dancing with Lucy, Mimi grabs the money, which is more than enough for two hands to carry. Then the shot is cut to her running out with a small nicely stacked handfull of bills. The real slip-up is the shot then cuts to a larger view of the club and you can see some bills fall to the floor once Mimi runs by. One of the extras notices and tries to kick them out of the way, but not even discreetly.