Gabe Aul of Microsoft addressed this question back in early May, his response was posted on Twitter.
Insider Question #1: @GabeAul Will you be able to upgrade to the #Win10 RTM version for free with #insiders preview?
Gabe Aul: Yes (WC users keep reading...)
Insider Question #2: @GabeAul @tao_huan Please make it clear. Do you mean free licence and activation or just free to upgrade?
Gabe Aul: "You'll have to be upgrading from a system with a valid license for 7 or 8.x"
So there you go, no FREE upgrade for Insiders UNLESS you have a valid Windows 7 or 8 license.