Ok...

Gamma World 2nd Edition was perhaps the best of the Gamma World editions, and it's actually somewhat scarce compared even to 1st Edition.  I've seen this one go as high as $30, with box and all components.

Gamma World 3rd Edition is practically unplayable without the errata, and even with the errata, was rather cumbersome.  Consequently it was not terribly popular.  I've seen it though go for about $15 to $20, which is a bit surprising given how sucky the mechanics were.

Metamorphosis Alpha I'd be interested in purchasing.  They tend to be in the $25 - $40 range for that printing, as it's a later printing.  Without knowing the actual condition of the book though, all we can give are ranges.