GRB was, indeed, based upon Robo Punch. I've always theorized that GRB was originally intended to be a larger scale version of RP with smaller scale Micromen that became so cool and different from previous Microman toys that it got it's own toy series (thus being the only released Diaclone for nearly a year).
As for scarcity and prices, I'm ashamed to claim ignorance on that one. Not being a Microman collector, I've never checked the auctions. Have you tried asking over at Microforever?