I think it will always be free as an upgrade for 7 and 8 users. New licenses and vista/xp users are the ones who would have to pay.
I would not mind putting it on this pentium e5300 I have here but I dont want to use my Dev copy on it, by the time I go back to school to my i7 it would have already come out. I like how this 5300 got a free upgrade to 7, and will now get the free upgrade to 10 though. Dont want to mess with my laptop though.
Hmm....