It depends what you like! I know a male spider or pet quality pastels are pretty cheap. Look around on the world of ball pythons morph list and get initiated.
It's also not ike he ingest a ton of substrate when they eat. The same thing happens in the wild, they should pass everything just fine, especially if they are small peices. Really- do you think ball pythons would exist if everytime they ate some dirt, they died? Lol he'd be fine. You could even have a slab of rock in there that you could put the rat on so he didn't eat in the substrate.