According to today post on the Windows Team Blog all current users of Windows XP and above will be able to upgrade to Windows 8 Pro for $39.99.
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. As and added bonus for those of you that want Windows Media Center you'll be able to add it for free through the “add features” option within Windows 8 Pro after your upgrade.
When you use Windows.com to purchase an upgrade to Windows 8 Pro, the Windows 8 Upgrade Assistant makes upgrading simple by walking you through the upgrade process step-by-step from purchase to download and then of course installation. The Windows 8 Upgrade Assistant will check to make sure your PC is ready for Windows 8. It will provide a detailed compatibility report that lets you know of anything you may have to address before or after the upgrade and outlines actions to take.
After your download finishes, you can choose to proceed with the upgrade (“Install now”) or install later either from your desktop or by creating your own media. If you choose to create your own media, you will be able to create your own bootable USB or .ISO file which can be burned onto a DVD for upgrade and backup purposes. If you prefer, you also have the option of purchasing a backup DVD for $15 plus shipping and handling.
If you aren't too big on digital downloads then Microsoft is going to make available a physical DVD, however the cost of the upgrade rises to $69.99. Which honestly doesn't make sense given the cost of the back-up DVD and considering how easy it is to create your own hard copies!
To note, this is a promotional price that will run until January 31, 2013. If you want to get the cheap upgrade price, you best act quickly and Microsoft has not said what the cost will be after January 31, 2013. Microsoft has not given word as to when Windows 8 will officially be available but they did hint that they are moving "closer to general availability."