I don't breed my own mice so I can't speak to the cost of producing your own feeders versus buying them. I do know that if I had to spend $15 per feeding I'd definitely explore other options.
Have you tried your Borneo STP on stillborn rabbit kits or chicks from local breeders? These often are inexpensive and if your STP won't eat them your king snake will, should you wish to give it a try.