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Cost is more than $100 if MS pricing doesn't change. But why bother? You have 2 options. First the "family" pack in 3 upgrades for Home Premium. But anything more and you might as well get volume licensing [I think you need to purchase 5 or more licenses.].
That said, I'm recommending those I know not to go to Windows 8. If they have XP, then purchase a Windows 7 system by October [assuming their system can't be upgraded]. Non-touch screen users will find the Metro interface a pain in the butt. [Why do we need to learn shortcuts in Win 8 but didn't in Win 7?]