Brandon LeBlanc, Microsoft:
We set out to make it as easy as possible for everyone to upgrade to Windows 8. Starting at general availability, if your PC is running Windows XP, Windows Vista, or Windows 7 you will qualify to download an upgrade to Windows 8 Pro for just $39.99 in 131 markets. And if you want, you can add Windows Media Center for free through the "add features" option within Windows 8 Pro after your upgrade.
By comparison, the Windows 7 Pro upgrade is $199.99; even the less full-featured Windows 7 Home Premium one lists for $119.99.