Puffed rice is like rice krispies. Cheerios are also good.

I feed my mice a mixture of seeds, grains, nuts, fresh fruit and veggies, and the occassional meal worms and crickets, meat scraps, and eggs.

Grain is cheap. You can buy 50lb bags of it at feed stores for $10-$15 bucks.
Seeds, you can get from Wild Birds Unlimited for great prices. Seeds from pet stores can cost $10 a pound, but seed from WBU is about $10-$15 for 20 pounds.

I also recommend dog food, in the mix. Holistic is better, because it has no corn or grains in it. Corn is virtually useless for mice.

Some of this is more or less unnecessary for a feeder breeder, but I breed for show, so my mice need to be super muscular, and large.

I give some to my neighbor when I cull, for feeders. He is amazed sometimes how large my mice can be. I have mice that, from nose to tail tip are as long as my fore-arm, about 12 inches long.