I bought a fairly good condition, complete grenade launcher set (cup, wrench, sights, pouch, etc) a number of years ago. Can't remember what I paid exactly, but it wasn't particularly cheap. Eventually sold it about five years ago, took a pretty good bath on it. Had it up on collectors sites for months, no interest. Finally wound up selling it for.... it was just under $1000 I think.
Since I got into reenacting, I've been looking for a replacement set. I agree, they seem to be not worth nearly as much as they used to. The most I've seen a complete set sell for lately, not on dealers website, was in the same $1000 ball park. Without the rear sight, I've seen them go for as little as $500. Relic grade ones can hardly be given away.
They're neat and all, but I'm just not sure I want to put out the money for something that is essentially a display piece.