Microsoft just announced that Windows XP, Windows Vista & Windows 7 are all elligible for an upgrade to Windoes 8 Pro, for just $40. That is between half to a quarter of the usual upgrade price.
Microsoft is also providing Media Center for free to those who take advantage of this upgrade offer.
That upgrade price will apparently be avaible only on Microsoft.com in 130+ countries but a retail pack will go on sale for $70. This price difference gives retailers some margin and Microsoft some manufacturing and distribution cost recovery.
You can read their blog if you want more details http://windowsteamblog.com/windows/b/bloggingwindows/archive/2012/07/02/upgrade-to-windows-8-pro-for-39-99.aspx