Gabe Aul from Microsoft has re-confirmed that Windows 10 RTM version will be a free upgrade for all Windows Insiders. Though you need to have “valid Windows 7 or Windows 8.X licenses” that most of us would certainly have before upgrading to Windows 10 Technical Preview.
Gabe Aul had earlier revealed that Windows Insiders will have option to stay with RTM build, if they wish to choose it or will keep getting newer builds, if they want to continue with their participation in the preview.
Also, it seems Windows 10 will be the last Windows major version as Windows will be continuously updated going forward rather than big releases every few years.
@farhan_kauser Windows will be continuously updated going forward, rather than having big releases every few years.
— Gabriel Aul (@GabeAul) May 11, 2015