Check sites like kijiji, you'll find cheaper rats than at chain pet stores. If theres a local reptile store, ask to see their live feeders if they have any, some of my friendliest rats came from feeder bins since they were bred by feeder breeders who don't put up with aggression (and you shouldn't either).
I keep most of mine in 105qt wide tubs, with 2-4 rats per tub and offspring. You have to add the hardware cloth yourself but to me its more fun than a rack system.
I throw in empty cardboard boxes, broccoli stems, Milkbones, basically everything they can eat/chew/play with that won't hurt them.
Also, depending on where you're keeping them and how many you have, I'd highly recommend making a cheap air purifier, for yourself and for your rat's health: http://ball-pythons.net/forums/showt...r-for-about-25