At the end of this month Microsoft will stop selling Upgrade Packs for Windows 8.x which means users are no longer able to update to a Pro version with Media Center. As of October 30, 2015, the Windows 8 and 8.1 Pro Pack (which upgraded Windows 8 Home or 8.1 Home to Windows 8 Pro or 8.1 Pro, with Media Center) and Media Pack (which added Media Center to Windows 8 Pro or 8.1 Pro) are no longer available for purchase from Microsoft.
Currently the Windows 8.1 Pro Pack sells for $159 on Amazon, adding Media Center to Windows 8.1 Home and making it Pro currently costs $99.99 when purchased from Microsoft, When you're already on Windows 8.1 Pro, the update with Media Center is $9.99.
Previously purchased feature pack licenses will continue to work, and valid feature packs may still be available for purchase from third-party retailers. Media Center is also no longer available in Windows 10 but there are many free and open source alternatives such as Kodi and MediaPortal.