I'd love to know where we buy these $500 non-Apple iMacs
In pieces at your local computer parts store. Sony is a bad example, they charge a premium for their hardware design as well.
I've run OSX86 on $300 Dells, back in 2007 or so.
edit: oh, unless you're looking for the all-in-one format, sure, those are harder to find. I've got a $400 Dell all-in-one here that might be worth a shot, but I think that particular form factor is overrated and would prefer a laptop.