I just got a ATI Radeon 9000 64mb Mac edition card off ebay for $23 USD. Those seem to be the cheapest and latest. Its seems that people still want $60-100 USD for the 9200-9250 cards for some reason.
You could also find a PC version of these cards and flash them yourself. I just flashed a Radeon 7000 card. It was pretty straight forward.
