Boxed copies of Windows 8.1 are now available to pre-order from the Microsoft Store. As previously announced, a standard version of the updated OS will cost $120, while the Pro edition will cost $200. Pre-ordered copies will ship on October 18, the same day that the update arrives in retail stores.
Those prices above only apply to users who are updating from Windows 7 or earlier, though — if you’re already a Windows 8 user, Microsoft will make Windows 8.1 available as a free download on October 17. Microsoft does warn Windows Vista and XP users not to buy Windows 8.1 directly, though; instead, it recommends purchasing the regular copy of Windows 8 and upgrading to Windows 8.1 from there.
Source: Microsoft Store