The Windows 8 Team blog has announced the price of the Windows 8 Professional upgrade. For just $39.99, you can upgrade your installation of Windows XP, Vista, or 7 to the best and most feature packed Windows 8 version. After checking program compatibility and downloading the update, the Windows 8 Upgrade Assistant will install the upgrade.
This promotion is only valid until early 2013. If you want a physical disc, expect to pay $30 more.