When buying a chair for comfortability and being ergonomic, usually you look for these things:
* Arm rests should keep your elbows bent 90 degrees
* Adjustable seat height so you have feet flat on the floor
* Adjustable back rest, so you can reach keyboard/mouse without putting unnecessary strains on eyes, arms, spine etc.
* Adjustable seat depth, so you wont be inclined to pop forward - meaning you lose the benefit of back rest.
* Lombar support, lower back adjust.
With those things in mind, you have to physically go into a store and try before you buy. Why? Because every body is proportionened differerently and every chair will suit you differently. Some cheap models may support you better than a more costly one. While the costly ones usually have more adjustability, you may find a 25 dollar chair that suits your needs perfectly for your body.
So all in all, try before you buy and safe huge amounts of money.