Michelle contacted me for help/comment, and this was my reply, with some minor changes made on re-reading:
Hello Michelle,
I read this thread when it first started and noted Adam's one word answer at the time. Often the less said the more it means I find!!
I my experience valuing glass by people who are usually artists is always difficult, but I'm afraid I feel strongly that Mark is wrong to suggest you use the Art Sales Index (sorry Mark). It talks about the work sold by artists as paintings, or sculpture and is therefore definitely only a means of finding an upper estimate for a piece of glass - possibly. It is used to compare like with like, so for instance, a picture of a given size and of a given topic will compare similar in price to one that has similar dimensions and subject matter - but not if either is different. Also the price may change according to when, or where, an item is sold. Yet all are what the artist might be recognised and collected for.
Essentually you should be comparing like with like, or at least similar with similar, since the markets for art are usually considerably different to those of glass in my experience I would say that although it may help, it is only a very rough estimate for our purposes.
Glass therefore becomes an academic 'add on' to their work - especially if it is sold as something to help promote an exhibition or show. This seems to apply however aesthetically pleasing an item might be.
Just as estate agents compare similar houses in a given area to gain a reasonable estimate of what another house might be worth, so you will need to do with these glasses. When it goes outside those parameters experience kicks in, and becomes highly relevant.
Do not assume that they are worth what a picture by Hermes is. There is precedent for this proposition with, for instance, the work of John Hutton, even though many of his vases are individual items - and he is a glass artist/engraver, but on a large scale, such as Coventry Cathedral. Certainly, it is also true of the work of the artists who designed for pieces by Stuarts at the seminal exhibition held at Harrods in 1934.
The long and short of it is that providing you make a fair profit, then don't worry if you don't get every penny it 'could' be worth. There's an old saying in the antique business "always leave a profit".
Nigel
PS. Silly me!! Of course I would be very interested to see them in the flesh at Cambridge, so do pop by the stand Michelle. Kind wishes, N.