Upcoming changes to Windows 10 Insider Preview builds [UPDATED 6/22] @ Windows.com
It is a free upgrade for current users of Windows 7 or 8.
Those who just installed "preview" can keep using free preview versions, no limit, or purchase.
Windows 10: Is it free or not?
"The good news is anyone in the Windows Insider program (the official name for Microsoft's public beta of Windows 10) can stay on it indefinitely. Those users will continue to get preview builds as Microsoft keeps updating Windows, including one that Aul described as "for all intents and purposes the same build as what other customers will get" on July 29, the official launch date for Windows 10.
If you want real-deal Windows 10, and don't want to deal with all the bugs that accompany preview software, you'll need to break down and buy a Windows license. Of course, for anyone running Windows 7, 8 or 8.1, you're entitled to a free copy of Windows 10. Anyone else will have to pay, starting at $119."