My opinion - what a seller is willing to sell it for and what a buyer is willing to pay.
Some other thoughts on the subject can be found at
"How Much Is My Machine Worth?" Question Answered
Another thought is if an antique dealer says it is worth a certain amount ask if they will buy it for half of that amount. I sincerely doubt they would. Also ask if they have sold for that amount or how long they had sitting in their store.
Janey - Neat people never make the exciting discoveries I do.