How much is a copy of 8.1?
It's going to be about the same as Windows 8.
Just think of 8.1 as a service pack. It's free if you already have the OS and will be on Windows Update.
In addition to this, Windows 8 DVDs will be discontinued and Windows 8.1 DVDs will come out instead. So if you are buying the OS new, you may as well buy the 8.1 DVD when they come out on Friday.
It's exactly like Windows 7 SP1 and every service pack for a long time. You will be able to buy the update integrated into the OS installation media. You can buy Windows 7 with SP1 DVDs for example.