Windows 8 reached RTM on 1st august as well as it reached a few early adopters who have access to TechNet or MSDN subscription on 15th august.
As we already know that this time around Windows Media Center will be available as Add On Feature (paid feature) for the Windows 8 users,though the early adopters who upgrade their Windows 7,Vista or Windows XP to Windows 8 Pro before January 31st 2013 will get the feature for free.

Image Credits[Windows8Core]
There is still no official word on how much the add on will cost after the upgrade offer.But according to recent reports coming from Windows8Core, the Windows Media Center add on will cost $12.79,which they discovered during the installation process of Windows Media Center from Windows 8 RTM.
As you can see from the above screenshot,which indicates that the Windows Media Center add on will cost $12.79 for the users from the united states.The article contains a step by step process to upgrade to Windows Media Center from Windows 8 Pro.
Though at this moment there is no confirmation from Microsoft about any such upgrade plans,but we can expect an official word in coming days as we are a few weeks away from the official public launch of Windows 8 which is scheduled for 26th October.