If you have more realistic amounts of money to spend on a chair I would budget £100-£200, you should be able to find a decent chair in the £100-150 range, any lower than that and I honestly thing the quality starts to dip considerably.
The most crucial things I think you have to consider are:
- Padding/material (the more the better with the former, chairs that look like they have 'layers' of padding generally seem to hold up well and are far more comfortable)
- Height of the chair (consider where your neck/head will be when you decide you want to lean back)
- Lumbar/lower back support
Forget chairs that allow too much freedom of movement like rocking back and forth, these will do nothing for your back and just encourage lazy slouching and poor posture that will ultimately make any back issue worse. Also consider the overall adjustable height of the chair and clearance of your desk.
My chair is one of these:
http://www.staples.co.uk/furniture/seating/executive-seating/leather-and-manager_s-chairs/veronesi-leather-faced-executive-chair-walnutThat was originally selling for £180 but I found a bloke on ebay with one virtually new and virtually unused that he sold to me for £100. Bloody bargain, I've had my arse in it for 6 months solid virtually, backache and fatigue I used to get is all but a distant memory.
If you want one similar just keep checking ebay, they seem to come up often...If you work at a computer all day there are a few things you should never be cheap on IMO, the two most important of those is your monitor you stare at all day and the chair you sit in all day.