Well, this is kind of interesting in a roundabout way, because what is the value of that book? I might pay Bill's $20 (might), but I don't think I'd pay David's $40. The way I look at that mess, the only thing of value is the signature, independent of the book, and I don't really know what that's worth. It would be a different story if it was a rare book.
There's another telling thing, and that is that a school bought a signed copy, which was more expensive, presumably because the unsigned first hardcover was sold out. Those trade hardcover firsts are more rare than the signed firsts because so many of them went to schools and ended up looking like this one (I offer that up for the newbies in the audience, as I know many of you are thinking, "No shit, everyone knows that"). It's food for thought, mobsters!