Take it from someone with half a hardware store embedded in their spine - don’t go cheap on the chair! What good is saving a couple hundred on a chair if you wind up needing more visits to the chiropractor or worse, if a bad chair causes physical damage like herniated spinal discs and radiculopathy?
I’m still trying to find the best chair for me, but I’m leaning towards the Aeron. Another strong contender for me is the Steelcase Leap. Both are adjustable in several ways, but the Leap has a couple more options like in/out and rotation of the armrests in addition to the usual up and down. Both can be had for less than a grand, but not by much.
You can get good prices on either at online vendors like sit4life.com if you want new, or craigslist and eBay if you want to go for used, but there’s no substitution for sitting in one for a day. Try to find an office furniture dealer (not a big-box office supply place like Staples or Office Depot) and try them out. You may be lucky and have a used furniture dealer nearby - just watch out for chairs that are used up, not just used. Wherever you buy it, look for the ability to swap it for something else within X days if it turns out to be a bad fit. What may feel nice for five minutes may cripple you after four hours.