OK, I just got involved with this whole MAME concept but I've got 6 years experience with Mars Bill Validators.
It's very easy to set up a BV on your machine if you already have a coin acceptor working. Lowest cost one to pick up would be the AE2411-UXE.
This would take $1, $2 and $5's. and replace the "X" with 2,3,5,or 7. This number being the number of bills it will hold in hundreds (eg 200, 300 etc). This will most likely not be an issue but the larger the number the deeper the validator will stick into the cabinet.
From there you will need to provide 110v to the validator and wire the output to the coin in switch. Then you can set the validator to give 4 pulses per dollar and the PC will just think that 4 quarters have been inserted.
If you decide to do it and have any questions about the wiring just let me know. I can give you move exact pinouts if needed.