I think historically glider prices have been extremely stable, you basically get your original investment back when selling (minus inflation). I just selled my glider after 10 years of use with same price as a paid for it, and bought another one for the same price original owner paid 10 years ago.

Glider popularity topped at early 90's, since then number of pilots have dropped significantly. And even them market of used gliders have been remarkably solid. Maybe during last decade or so price of some pure gliders has dropped a bit, especially if glider has no engine or does not have particularly good price/performance ratio, and ASW27 most certainly has neither of these qualities. Nothing to worry about, just drop your price a bit.