About 5 of my buddies and i went in on a bulk purchase of locks a couple years ago. We got them down to about $2/each and now all of our machines use the same key. Anytime a new machine enters the mix the lock is put aside and saved for when the machine leaves the group.

Coincidentally our locks matches the ones used in newer Stern backboxes so we can open any new stern and find our key.

We also take a lot of games to shows and events and carrying 1 key makes it easy to repair machines.