I think a huge part of the equation is the history of the unit. Where has it lived? How many owners has it had and how have they cared for it?
There are so many little details that contribute and most of them are subjective and subtle - some things are really important to some people but trivial to others.
I read and learned a lot from the very wise paper on the subject, referenced by rcmenz above: you'll find it here:
This site is an invaluable resource to the vintage enthusiast, in my opinion. I'd spend lots of time there if you haven't already!
Anyways, I came away from that reading quite reconciled to the range of prices being asked.
Heh heh, if nobody is willing to pay what a seller is asking , then it's unreasonable..so of course the converse would also hold true.
Just my 2¢ - not by any means an informed opinion.