You need only type "Is Windows 10 free?" in your favorite search engine to be met with thousands of articles all saying different things, "it's free at last!", "it's free as an upgrade!" and "it's free as an upgrade for Windows 7 and 8 users!", the latter being sort of correct.
So lets clear it up right now, by looking at official sources. It comes down to this.. it's free for Windows 7 and 8.1 users only in the first year, this much has been confirmed by Microsoft in an official blog post way back in January.
Today was a monumental day for us on the Windows team because we shared our desire to redefine the relationship we have with you – our customers. 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.* - (emphasis ours)
The little asterisk points to the 'small print' at the bottom of the page stating "Hardware and software requirements apply. No additional charge. Feature availability may vary by device. Some editions excluded."
Meanwhile, Gabe Aul has also reiterated this in tweet responses:
It can't get any clearer than this, though this leaves is us to wonder what the OS will cost after the first year, either as an upgrade or for the full version. Microsoft has yet to announce those details, and we'll be there to cover it when that happens.
The next question on everyone's minds is this, will it be free for Insiders? And this is a question that has yet to be answered. Gabe has said several times that Insiders will be able to upgrade all the way to the final RTM, but license validity thereafter remains unclear. Will Insiders upgrading to RTM from a clean install need to purchase a license to use the OS or will be they receive free licenses from Microsoft?
To put more emphasis on why Insiders are expecting a free copy of Windows 10, you need only to look back at the Windows 7 Technical Preview on Connect, Microsoft's older preview program. Several thousand users were gifted the RTM version of the OS once they submitted feedback and bug reports. Those that didn't, but were still in the Connect program did not receive the RTM gift. Microsoft originally didn't plan to gift Windows 7, but had a change of heart after backlash from testers.
Unfortunately, it does not look like the Windows 10 RTM will be gifted to Insiders, at least for now. This could change closer to the release, right now however, all we know is that Microsoft is saying that a valid Windows 7 or 8.x license will be required. And that officially answers that.