Ran into a little snag today. Looks like I'm not going to be able to use the stock coin counter as its voltage doesn't match the output on the ArcadeSD. Options are to replace the coil in the stock counter, or use a whole replacement counter. I'll add one to the shopping list I guess.
I know its a little late but I think you could have just used a NPN resister. It would have run less then $1. Not 100% sure it would have worked out here. I only know about them from the instructions from when I built a 1watt CNC laser.
It would have look something like this, ground from your regulated power supply to pin one, +5 from your counter to pin two, and +12 from you power supply. You might have had to add a resister to pin two but that is simple. The way I under stand it, it works like a gate that opens when there is a small positive current applied to pin 2 and the higher voltage circuit is aloud to close.
Just thought I would share in case some one has similar issues. I know this build will inspire others. This is coming along great.