Well, lots of these things seem to have turned up. I guess that means it's safe to say these aren't rare and valuable.
They seem to have been around for awhile, too. I think Book's theory, that these were given out with invitations to parties, to use as entry passes, is interesting and feasible. It sounds vaguely mysterious, exotic and secretive and I can see that sort of thing having some appeal to bored "high-class" people in times past.
How much would they be worth today? A couple of dollars, I'd guess. Maybe a little more, if the story behind their manufacture and use can be determined for certain.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis