!. For each model that you can think of, there may be 50 or more years of production. For that reason, a White Hunter produced in 1962 should be worth more than a White Hunter produced in 2002.
2. Buyers and Sellers have to agree on condition. What is Mint Condition to some is only Near Mint to others. Condition has a heavy influence on the price of a knife.
3. Rarity is also a big factor. Many people cannot agree on price because in their mind it is more or less rare than what other person may think.
4. Lastly, no one really wants to keep a public record of what they got for the last one, hoping to make more on the next one.
Having said that, there are resources that, for a monthly or annual fee, you can obtain information about past sales at auction for most knives. The problem is, unless you are constantly using the service for multiple items, it is hardly worth the cost. Worthpoint comes to mind as a example of this service to sellers and buyers. The usual client of these services are dealers in antiques who cannot possible stay on top of all things sold in their stores.
When it comes down to it, your knife is worth what another person is willing to pay for it. Sometimes the price is all over the place and fluctuates wildly.
My suggestion to people who ask the question is for them to evaluate auction sites such as eBay and get an idea of what a similar knife in similar condition has recently sold for. Over time you can an idea of the sale trends for your knife.