While Microsoft is not sure whether it wants to give Windows 10 for free to people using pirated copies of Windows 7 and 8, the company will provide the final Windows 10 build to its testers for free. Gabe Aul, GM of Microsoft's OS Group, posted on an official blog that all users running build 10130 of the Windows 10 Insider Preview linked to a Microsoft account will get the final version of Windows 10 for free.
The blog post states:
“Windows Insiders running the Windows 10 Insider Preview (Home and Pro editions) with their registered MSA connected to their PC will receive the final release build of Windows 10 starting on July 29th. This will come as just another flight. I’ve gotten a lot of questions from Windows Insiders about how this will work if they clean installed from ISO. As long as you are running an Insider Preview build and connected with the MSA you used to register, you will receive the Windows 10 final release build. Once you have successfully installed this build, you will also be able to clean install on that PC from final media if you want to start over fresh.”
There is a kink in the matter however, earlier the company stated that “As long as you are running an Insider Preview build and connected with the MSA you used to register, you will receive the Windows 10 final release build and remain activated.” Since then the term “ “remain activated” has been removed and this has been added "It’s important to note that only people running Genuine Windows 7 or Windows 8.1 can upgrade to Windows 10 as part of the free upgrade offer." This entails that users connected to the Windows Insider program will get the final build but may have to pay to for some features or to get it activated at some point (or it could simply mean that Microsoft is trying to cover its bases with legalese and making it appear that while it does not condone people upgrading to Windows 10 for free, it won't stop them from doing so).
Aul also added that the Windows Insider program will continue even after the final release of Windows 10 and users registered with the program will continue to get future updates. The post also says that Microsoft will start working on the next iteration of Windows as soon as Windows 10 is released.
Source: Windows blog