I think this is as good a place as any to ask (based on your membership level).
Unfortunately knives aren't like stocks, there is no "knife market" online where a person can look up the current value of a knife at any given moment. The best you can do is do an internet search for that knife and try and find the most recent "for sale" listings where the knife actually sold (like ebay).
Of course, like most things, monetary value comes down to what someone is willing to pay. Someone can tell you that your knife is worth say $800, but if you try and sell it at that price, and no one is willing to pay it, then is it really worth $800?