I use newspaper, aspen, and cypress mulch. Newspaper for picky feeders that I feed in their cages. Aspen for low to medium humidity species, and cypress mulch for higher humidity species although sometimes I just use aspen instead.
If you need cheap, use newpaper. Ask a coffee shop if you can have old newspapers if you really really want to be cheap about it. I live in an apartment building, and I just pull them out of the recycling bin in the laundry room.