According to WinBeta on Sunday, Windows 8.1 will be generally available on October 18.
Microsoft has already started taking pre-orders for Windows 8.1. Windows 8.1 cost $119 and Windows 8.1 Pro cost $199. If you are running Windows 8 already, the update to Windows 8.1 will be free.
If you are using the Windows 8.1 Preview, upgrading to the final release of Windows 8.1 is possible. All your apps will need to be re-installed once you upgrade to the final release though. You will only be able to transfer your data from the preview to the final release.
Windows has a few reasons why you will 'love' the new upgrade to Windows 8.1.