Murano glass buyers are strange animals. Unless it is rare, even a little damage can reduce the value to near nothing. It is not that the value dips that much on paper, but almost no one will buy it. The good thing is the person buying the slightly damaged piece is buying it for themselves and not for resale.
Do you know who did the face? That will be what determines if it retains any value. (Appraisers say a small chip takes 25% off the value. That may be so, but it becomes very slow to sell, so it is hard to say.)