alright dude, heres what you need to do. ive had this same thought going through my mind...."it should be free, but man it would be cool if i could make some money off it". well im buying some ultimate mechs from TheNasty here on the board (go to the buy/sell/trade forum) which accept all coins. yes, all coins, even tokens. on my cp i have coin buttons for each player, but im also going to wire my coin door to accept coins using those mechs. and i will encourage a "donation". i know i wont make much, but hey, it will buy me a coke or something every now and then.
example: my friends come over, and ill say "alright, you can play as long as you want, and as many games as you want, just give me the change you have in your pocket." which would be like either a dollar, or two cents. its just spec, but it could happen. and if nothing else, just say, put a quarter in and you can play forever. just a thought.