Value, as with most things, depends on the subject and condition/clarity.
Legendary has auctioned several over the past couple of years. You could check out their archives.
I personally think they are often undervalued. They are extremely delicate, remember they are made of glass. I also think they are really special since at one point in time the glass negative was within a few feet of it's subject. Not very many pieces of our treasures and valuable cardboard can argue that it was ever within that type of proximity of it's subject.
I only own one. Waner in 1932. It has the original filing envelope and notes from The Herald. Picture is showing it held up to the light.
