The Insider Preview releases have an expiration date, but if you continue to accept new builds, no cost is incurred. Build 15914, for example, expires in May of this year.
If your hardware has been activated by Microsoft as a result of a digital entitlement due to taking advantage of the previous 'free' upgrade, if you bought a new machine with Win 10 installed, or if you bought a copy of the Win 10 to install yourself, you can revert to the current production release at any time and incur no additional cost.
Simple participation in the beta program on a machine that has not earned a digital entitlement will almost certainly require you to obtain a license at some point. At one time, it was a legitimate path to a free Win 10 license, but that was then, this is now....
Disclaimer: There have been, and probably still are, some sneaky ways around this, but you'll have to do the detective work to find out for yourself.
Thanks DJ- I want to keep everything legal- not looking for loopholes-
[[If your hardware has been activated by Microsoft as a result of a digital entitlement due to taking advantage of the previous ‘free’ upgrade,]]
That’s the thing- i only tried it briefly in a VM without activating it- it kept nagging to but i never got around to it- Then next thing i know- they discontinued the free upgrade and i only found out about the discontinuation o n the day they discontinued it- just a few hours left in the day too- unfortunately- but that’s always been my luck- had i learned about the discontinuation a few days before, i could have decided whether to register it or not-
oh well-