You can get Windows 8 for 15 bucks:
http://www.windowsupgradeoffer.com/en-US

The catch is you must have a Windows 7 PC purchased in 2012. (the Year dropdown select field implies this offer will be good in 2013 as well)

The site does not check when your PC was purchased, thus a loophole. However, if your PC was purchased prior to 2012 and you take advantage of this offer, you will be in violation of the site's Terms of Use Agreement.

I built myself a new desktop today, installed Windows 7 and it's downloading Windows 8 Pro as I type this. I never do upgrade installs for operating systems, but in this case it was a clean install of 7 with nothing else installed except drivers.