If you're buying 2nd hand, Senator chairs arent that expensive and are comfortable in my experience.
In the offices i work in we use:-
- Senator Freeflex Mesh
- Senator Ecoflex
- Herman Miller Aeron
- Herman Miller Mirra
Comfort wise i rate them in the order they're in above, although the Internet loves the Aeron.
I bought a mint 2nd hand Freeflex Mesh for around £100 that I use at home, Ecoflex's sometimes go for less.