Based on the blog entry from Brandon LeBlanc the Windows 8 Upgrade (via Windows.com) will probably be available for $39.99. This is quite a smart move by Microsoft as you get Apple’s Mac OS X Snow Leopard from their store for about $29.00.
If the upgrade really works from Windows XP up to Windows 7, Microsoft might be possible to shift many users to the next generation of Windows. While the price for previous Windows upgrades was quite high, many users kept their former version of Windows until they bought a new personal computer, obtaining a new OEM version of Windows.
Nowadays, computers used at homes definitely last longer than the average Windows life cycle and quite a lot personal computers outside in the wild might see at least two version of Windows until being replaced.
Therefore, personally I welcome this move by Microsoft, considering to upgrade even more than one of my licenses to the new Windows version. Even though, this might be a strategic move by Microsoft to keep market share, existing customers definitely gain by the relatively low prices by upgrading even from previous Windows version. This might be quite an advantage as one has to buy each and every Mac OS version on the way upgrading from previous versions.