The value of such an item depends on quality, construction materials, historical accuracy, visual appearance, scale and of course market.
If you are the seller it worth more than if you are a buyer.
Do you have any close up pictures of it.
Oh an informal rule that one of regulars here has requested. If you take pictures of your cannons or mortars include a powder can in the picture for scale. It has became almost a board tradition. In fact once you become a board regular you will be severly chastised by the board members if you forget....they are merciless on that!!!