I would go on the gun auction sites and see if I could find a comparable rifle being sold and see what they are asking. Then see what kind of bidding is being made. A lot of times, what people think their rifle is worth and what they actually sell it for are much less.

Also if it is a flintlock you can add a little more to it. Was this one of the CVA Kit rifles they offered a while back? They were a nice enough kit. Not real expensive, and pretty nice wood.