a knife is only worth what you can sell it for in specific amount of time. Les Robertson stops by here often. He is probably the most active on the net doing that kind of than and he can give you an appraisal. I've no idea what he charges but I know it won't be free.
Another method is to watch eBay for several months and look for similar knives. I know you said it is unique but a knife is a knife is a knife and there is going to be something similar come through at some point. eBay isn't the last word in what a knife is worth but a sold price won't be that far off.