the sky, er.... the wallet, is the limit! lol.
i'm lucky in that i found an organic apple tree this fall. i processed all the twigs and skinnier sticks from the cuttings and i keep them in open containers on top of one cage. whenever i'm in the chin room i dole out the sticks, anywhere from 1 to a few for each of the boys. Rhino loves this apple wood, whereas Guss really isn't much of a chewer when it comes to wood and toys. he much prefers to wear his teeth down with hay and pellets, but does chew the odd stick or loofah here and there. i keep sticks in his cage all the time in case he wants to chew, but usually i end up giving them to Rhino untouched.
both boys also have oodles of hanging toys and other loose pieces of wood and loofah in their cages too.