Unfortunately determination of value is a relative, not an absolute. The definition of Fair Market Value is "the amount that a person, under no compunction to sell, is willing to sell a specific item for and that a person, under no compunction to buy, is willing to spend to purchase the item".
There is an absolute portion, which is the value of the pass to enter DL. However, when it was sold at a price above the admission part of the initial selling price was an estimate of what a person would be willing to pay for a rarity, as well as any other items which came along with the admission ticket.
The only way to determine the FMV would be to put it up for auction, possibly with a reserve set at what you think the value should be or at least so you don't have a loss on the sale.