I bought a HM celle and although it was a big step up from my cheapo ikea chair it wasnt anything like as good as an aeron. I eventually caved and bought one of those instead and I dont regret it. You should be able to find one within your budget second-hand pretty easily.
See I liked the Celle loads more than the Aeron. Even the lab spec one with the hard plastic seat. Mirra probably the comfiest though. Form over function? Hell no, they're butt ugly.
I bought a HM celle and although it was a big step up from my cheapo ikea chair it wasnt anything like as good as an aeron. I eventually caved and bought one of those instead and I dont regret it. You should be able to find one within your budget second-hand pretty easily.