This title block from Blogging Windows says it all:
The blog in question is from the Communications Leader for the Windows 8 team, and the spokesperson for the group’s blogging activities, Brandon LeBlanc. He says “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” (that means 131 different countries, and in 37 languages, too). Better yet, this offer also includes the ability to “add Windows Media Center for free through the ‘add features’ option within Windows 8 Pro after your upgrade.” A packaged version of the upgrade on DVD will also be available while this promotion remains ongoing (from the GA date through January 31, 2013) for $70, and buyers who want an install DVD can buy one for $15 direct from MS.
But wait, there’s more: according to the answers.microsoft.com forums on July 6, 2012, the answer to the question “Are Windows 8 Release Preview users eligible for the $40 upgrade to Windows 8 Pro promotion?” is “Yes,” as long as they own an underlying license for XP, Vista, or Windows 7 for the machine running Windows 8 RP. That said, you won’t be able to migrate apps and will generally do a mostly clean reinstall, as with other earlier transitions from Developer Preview to Customer Preview to Release Preview. Still a pretty good deal, though — at least for those brave enough to move to the GA release of Windows 8 when it finally arrives.