Windows 8.1 is a free upgrade for all Windows 8 users, but those upgrading from an earlier version will cost $119.99 for the standard version and $199.99 for enterprise-centric Windows 8.1 Pro.
If you clean installed Windows 8.1 Preview by using an ISO then you can update to the final version of Windows 8.1 using the Store today, but the system won't activate without a Windows 8.1 license key. Microsoft says existing Windows 8 keys will not work on systems that clean installed the Preview and update to 8.1. If you were previously running Windows 8 then you'll need to reinstall that copy before moving to 8.1 to ensure you don't have to purchase a new key.
Microsoft has released a large number of built-in app updates for Windows 8.1 today, so expect to see those appear following the upgrade. The software maker is also releasing a final General Availability update patch today for existing Windows 8.1 users and those upgrading, so if you have any limitations on your monthly broadband bandwidth then bear that in mind before you update. Finally, if you're a Surface RT user then the Windows RT 8.1 update is also available today and details can be found at Microsoft's support pages.
Update: If you're having trouble finding the update in the Windows Store, simply visit this article on your Windows 8 device and click this link.