Huh, I got Win 7 Ultimate for $7.50 by buying it at my college bookstore. I advise college students check their bookstores before buying it online, because they often have it cheaper. The only thing is, at many of these stores you have to ask, because they don't keep it out in plain site. You also probably won't get the retail packaging, just the disc and a serial.
Only downside for me was that I didn't get any choices whatsoever. All they offered was Ultimate 32-bit (which I wanted anyway, but still the ability to choose Professional or 64-bit would have been nice).