Head of Windows Insiders Gabriel Aul has confirmed on Twitter that all members in the program using the Windows 10 Technical Preview (or the Insider Preview as it is now called) will be able to upgrade to the RTM build of Windows 10 for free.
@tao_huan Yes— Gabriel Aul (@GabeAul) May 10, 2015
There isn't a mention as to when Windows will go RTM (Release to manufacturing), although it has been suggested that we'll see the build released to OEM vendors sometime in the month of June, allowing machines with the new OS to be available in time for the back to school season.
Source: Twitter (Gabe Aul)
Thanks Sachinmeet, Nicholas and Adam!