I don't know about the costs of feeding mice up to adult size, but if you have a smaller ball python the cost is next to nothing apart housing your two new pet rats. Frozen mice at my local pet store cost about $20 for a dozen. Our rat just had her first litter(flock? lol) of pups(and a HUGE one at that, 12 pups!) and they were large enough for food after a week and a half, and they feed off of the mother in that time.