Hey Brandon, the most cost effective food to feed them is a high quality lab block(Mazuri, Purina). I buy a 35lb bag at the feed store for $11, and if you are only breeding mice to feed to your corn snake, I would definately only get 1 male and 1 female. It wouldn't be cost-effective at all if you got any more than that.

I keep my 2 pairs in 10 gallons, on aspen. If they are on pine, they seem to sneeze alot more and just look kind of ragged. Sometimes some hair may fall of of their face if left on it too long. I just changed them out (from pine to aspen, my school keeps them on pine) and the male mouse that was sneezing, has stopped. So hopefully he'll feel better after a little while. I give mine all sorts of stuff to hide/play in, toilet paper tubes, paper towel rolls, kleenex, etc.

My 2 females seem to be getting a little fatter, I don't know if it's the food or from being pregnant, but they are a lil chubby

Also, about wanting one rat... If you were able to get one, you would have to spend the majority of your day with it, if the rat is alone. Rat breeders won't selll you a lone rat unless you already have a buddy for it at home. Lone rats can get depressed, and sometimes mean. Just informing you in case your mom every lets you get them. Here's a good website to read about them:
www.rattiusmaximus.com Chrissy is a very nice person and if you have questions about rats, just send her an email. I have 1 of her babies, Willard, a black rex dumbo boy, and he is one of the sweetest, kissiest boys we have.

Good luck with the mice though, they are funny to watch.