It recently came to light that the upgrade from previous versions of Windows to Windows 8 Pro would cost just $39.99 until January 31, 2013; a surprising bargain considering previous upgrades cost more than $100. With the deal, anyone can upgrade their copy of Windows XP, Vista or 7 to the full version of Windows 8 Pro. A physical upgrade DVD will also be available in retailers for $69.99.
At the time of the announcement it was unclear if you would be able to upgrade from the Release Preview to the Release to Manufacturing (RTM) final version of Windows 8 for this low price, but since then it has been revealed by Mary Jo Foley that this is indeed possible. Speaking on the Windows Weekly podcast, Foley mentions that the $39.99 upgrade deal also applies for those people testing Windows 8.
...[$39.99] is also the price for people who are testing Windows 8... I just asked to make sure and they're saying yes, if you have been testing the Windows 8 Release Preview you can upgrade for the $40 too.
This means that you can effectively get a full copy of Windows 8 before January 31, 2013 for $40 by simply downloading and installing the Release Preview for free, and then upgrading to Windows 8 Pro for the low price of $39.99. That said, it is good to see Microsoft is rewarding all those who have spent time testing Windows in the months leading up to the release.
Update: Brandon LeBlanc mentions on the Microsoft blog that to upgrade from the Release Preview you need "an underlying license for either Windows XP, Windows Vista or Windows 7". Luckily for most people, finding a Windows license from the past 11 years shouldn't be that hard, still enabling you to upgrade extremely cheaply.
It should also be noted that those who upgrade from the Release Preview will only be able to keep their documents and files, with previously installed applications not making it through the update process. Thanks for the update, lcg, in the comments.