How do you put a value on a colt dragoon circa 1800's?

Find a reputable gun dealer in your area, or better yet a real antique store or auction house. Depending on where you live I know that Bass Pro Shop has a very good departmewnt that deals specifically with rare and expensive guns. If nothing else I will give you 10 bucks for it. Without information such as modlel, serial number, condition... your question is far to vague to even consider answering.