We already know that Windows 8 users will get a free upgrade to Win8.1. The update will be available for download starting October 17. Now, Microsoft has confirmed how folks will be able to purchase the new OS on its own.
Win8.1 will be sold in a retail box and in downloadable form. The standard version will be priced at $120, while its Pro counterpart will run $200. Those prices are identical to what Microsoft charges for Windows 8 upgrades right now. Here are the requisite retail box renderings:
Microsoft's blog post notes that Windows 8.1 is "full version software." Unlike the similarly priced Win8 upgrades currently selling via Windows.com, it won't require a previous version of the OS.
Of course, Newegg charges only $100 for Windows 8 and $140 for Win8 Pro right now. That's for OEM variants of the OS that don't require previous versions of Windows, either. OEM copies of Win8 also include a Personal Use License that explicitly permits installation "on a computer that you build for personal use." I expect we'll see DIY-friendly versions of Win8.1 after the OS is released, probably at similarly discounted prices. In the meantime, you can pick up an OEM copy of Win8 to be eligible for a free upgrade to 8.1.
That Windows 8.1 Pro Pack on the right is what's required to upgrade standard Windows 8.1 machines to the Pro version of the OS. The Pro Pack includes Windows Media Center and is set to cost $100, just like its Win8 equivalent.