This will basically boil down to a judgment call on your part. Breeding your own rodents will save you money but mice stink, and I mean they STINK! Mice ammonia smells far worse than rats and mice take forever to get a decent size but since you feed small mice that really doesn't matter. You would only need like a 1.4 ratio for breeding so I guess it won't be so bad. I have to remember you only have 2 snakes.
On the other hand you only go to get rodents twice a month which isn't that bad and no rodents to feed, clean and water.
Personally I would find someone locally that bred mice and just buy from them, it would be a lot cheaper and you don't have to breed any rodents until your collection grows!