Why is it that most office chairs only have less than 3 inches of crappy cushion?/QUOTE]
So that you may maintain proper posture, more cushioning might mean that you sit more comfortable but also with an improper 'slouchy' posture that could be detrimental to your back in the long run.
I know it can be hard but I would spend more than 400 on an office chair, just think about the amount of time you spend in your chair and then it usually becomes a justifiable expense, especially in the long run. Expensive chairs from HM, Knoll and Vitra are not only better designed but will last a whole lot longer. One of my Eames chairs is 27 years old and the other 17 years, both look reasonably new and the fabric between the frame hasn't sagged one bit. I don't see most 400 dollar chairs do that. And besides that there is that oh so subjective thing of how pretty I think they are.
Oh and take a proper 'test-run' with the chairs, people's bodies are very different and one might find that for example the Aeron sits wonderfully whilst an Aluminium group is horrible or vice versa, or they might even hate or love the way they seat both equally. It's a very personal thing.