See the very interesting article "Upcoming changes to Windows 10 Insider Preview builds" by Gabe Aul in Blogging Windows for the latest details about Windows 10 and it's official release in about one month.
You can download the last version of Windows 10 technical preview - which is April Update (build 10130) - here for free: Download Windows 10 Insider Preview ISO June Update. The latest updates will automatically come via Windows Update.
IMHO it's not clear if members of the Insider Program will get Windows 10 for free as posted on the Verge in Microsoft is giving away Windows 10 to anyone who tests it:
"Microsoft's Gabe Aul confirmed this method on Twitter yesterday as part of a new blog post detailing some new changes to the way the company tests Windows 10. "As long as you are running an Insider Preview build and connected with the MSA (Microsoft Account) you used to register, you will receive the Windows 10 final release build and remain activated," says Aul."
But there's a brand new info by today on the theVerge article: "Update, June 21st 9:45AM: Microsoft has updated its blog post today and removed references to "remain activated." The stealthy edit isn't acknowledged, and we've reached out to the company for comment."
Also Microsoft has made clear that the RTM version will not include "all" features of Windows 10 and that functions will be gradually updated by Windows Update.
Anyway, looking forward to the availability of Windows 10!