You can look at what has sold in the past on eBay and try and average out the lowest and highest price for that deck. Also, see if you can find the amount of decks originally printed. For example the Red Fonatine's their were only 2500 decks printed and those are going for an average of $300 or more. So you have to figure, if their were 5,000 or 10,000 copies printed, then they wouldn't be sold for as much, because their are more in circulation and hence more to be sold by other sellers. It's all about supply and demand and how much someone is really willing to pay for something that sets it's value.