It is for FREE if you already have Windows 8 via the Windows 8 store. If you can’t see it there just yet, don’t worry because it will appear in time. It is a major update, so ensure that you have backed up any important stuff to external media before attempting to install it.
If you are looking to install Windows 8.1 over Windows 7 or Vista, it will cost you FULL price now. No offers this time around because any chance of getting Windows 8 cheap ran out many months ago. You should kick yourself if you didn’t take up the offer. Windows 8 was and is nothing like as bad as the Windows 8 haters make out. If you listened to some, you would still be fiddling around in DOS 5 and maybe, just maybe contemplating the move to Windows 3.1.
Microsoft is advising us NOT to upgrade a computer originally running XP. Personally, I think that you should be dubious about an upgrade of a computer originally running Vista, especially early Vista computers. Hardware drivers can be an issue and you should check with the computer manufacturer for evidence of Windows 8 drivers before going ahead.
Upgrading desktop machines is not so bad but be prepared to fit 3rd party sound, NIC and video cards which are Windows 8 friendly. Laptops are not good in this respect, and while there are external devices around, they seriously hurt portability, the whole essence of a laptop.
If you are running Windows 8 with a 3rd party start menu, it should still be there after the upgrade. Start8 certainly is but you will have to go into Start8 settings > Desktop and pick a custom icon for the start button. Not sure about how it might affect Classic Shell or any of the others, but you may have to make a similar change.
That’s about it for now. Good luck and have a good time with Windows 8.1..