Today, Microsoft announced the pricing for Windows 8.1 and it's no different than Windows 8. Windows 8.1 is $119.99 US and Windows 8.1 Pro will be $199.99 US. For those already running Windows 8, Windows 8.1 will be a free upgrade.
Full announcement here: Pricing and Packaging for Windows 8.1
In addition, if customers purchase a device that has the Windows 8.1 edition (that cheaper, non-Pro version) installed on it, they can later purchase a Pro Pack for $99.99 US to "unlock" all the Pro features plus Windows Media Center. Even for Windows 8.1 Pro, Windows Media Center is a separate purchase that must be downloaded and installed using "Add Features to Windows 8" function of the operating system. Windows Media Center provides the ability to play DVDs and other media. Windows Media Center continues to be priced at $9.99 US. Check out the Installing Windows Media Center portion of the following article if you're interesting in how that works: Playing DVDs and Blu-Ray Discs on the Surface Pro.
In addition to setting prices Microsoft has also announced a "shift" in what they are offering in relation to upgrades. Microsoft is making full version software available this time, providing installation bits that don't require a previous version of Windows to be installed. Microsoft states the change was brought about due to customer feedback where PCs need to be built from scratch, PCs require dual boot with Windows 8.1 sitting on different hard drive partition, and also to deploy in a Virtual Machine.
In the announcement post, Microsoft also bullet points a couple important notes about supported upgrades:
Windows 7 can be upgraded directly to Windows 8.1, however only user files will be retained. All desktop applications, including Microsoft Office, will need to be fully reinstalled, i.e. the upgrade will wipe out the applications.
Simply put, Windows XP and Windows Vista are no-win scenarios. Neither operating system version can be upgraded. Instead a brand new, clean copy of Windows 8.1 must be installed and the installation will wipe out everything on the hard drive including settings, programs, applications, and files. If you decide to go this route anyway, make sure to have proper backups of your personal files.
If you're running Windows XP and thinking about moving to Windows 8.1, it's probably time to purchase a new computer anyway. With Windows XP's end of life coming in April 2014, it'll just be cheaper to acquire a new PC with Windows 8.1 already installed.