When Windows 10 comes out this summer, it will be a free upgrade for Windows 7 and 8.1 users. But only during the first year. Whatever happens after that is anyone's guess now.
Microsoft confirmed this upgrade period in a blog post back in January [emphasis mine]:
We announced that a free upgrade for Windows 10 will be made available to customers running Windows 7, Windows 8.1, and Windows Phone 8.1 who upgrade in the first year after launch.*
That asterisk leads to a footnote saying feature availability could vary by device, some editions excluded, hardware and software requirements apply, and "no additional charge" [presumably for the upgrade.
Once you're upgraded, Microsoft will push new features and updates for "the supported lifetime of the device."
Besides this, we don't really know much about Microsoft's long-term pricing plan. After the first 12 months after launch, how much will Windows 10 cost? And will we have to pay for Windows as a subscription? (Forbes suggests this could be the case, but it's really all conjecture right now.)
Microsoft's blog post does say:
We think of Windows as a Service – in fact, one could reasonably think of Windows in the next couple of years as one of the largest Internet services on the planet.
Hmm, most of us don't think of our operating system as an internet service, but it makes sense for Microsoft to go in this direction--OS-as-a-service would be more profitable when people can no longer hold onto their beloved operating system (e.g., Windows XP) for a decade.
I guess the takeaway is to upgrade your system before the launch year is up if you don't want to shell out for it. But perhaps we shouldn't think Windows will be free forever after, at least until we know more.