I haven't looked recently, but a while ago there were a few genuine Aeron chairs (used) going on Ebay for $350-400, which is more than your target, but might be worth it.

Right now I'm alternating between a piece of shit Office Depot special and this


which has done wonders for my back, but gets tiring after about 4-5 hours.

I'm actually thinking about springing for an Aeron chair. They are pricey, but I've heard that they really do hold up exceptionally well and when I've used them I haven't found anything more comfortable.