A very, very good chair is
the Aeon chair, which retails for about $520; like an above poster said, look for one on Craig's list or eBay. They have been around long enough now, and enough businesses have gone under, that you should be able to find one for less. Balanced against medical bills for back pain, even the retail price is not such a bad deal.
To stay on the safe side, go to various office furniture stores and try out various chairs before buying. Stay in the various models for 10-12 minutes with a laptop. The store will understand.