Shrinkwrapped GW1 and GW2 sold a couple weeks back for $15 - $20, which could of course be unusually low. I've only recently started collecting Gamma World, but my observation so far is that the later modules GW3 - GW-whatever seem to go for more money in any condition (an example of the rule of value being inversely proportional to quality, evidently

), but I'm only buying 1st ed. so haven't really been following prices on those closely. I would imagine S/W adds about the same value as for other modules.