Posture has nothing to do with your chair, and everything to do with how you sit on it. I've literally sat in front of my computer since I was 9 years old, on anything from old kitchen chairs to the cheapest, crappiest big-box store models.. Actually, the only place I've ever sat on an expensive chair has been in the office, and it was not that much better.. It's all about how you sit..

I sit in many weird positions throughout the day.. lean back, feet on a box. straight up and down, legs crossed, lotus, and yes, I squat on it too.. sometimes on my knees, sometimes the chair reversed.. pretty much anything that keeps my head above the table.. totally privileged, I know :)

Hell, the last chair was thrown out after it literally broke apart, it and before it did, it was crooked to one side for a few years. I'm 35 now, my back is perfectly fine. It's not about the stuff you've got, it's about you.