Last time I had the boys over for a pinball day at my place I told them it was $1 per game and I kept a tab. If they won a credit I would either credit a dollar from there tab or let them play the credit . At the end of the day from 4 machines (and 5 guys playing them) I had made around $300.
It made no difference though because when it came time to settle up they all told me to shove it and went home