Your description of the piece is too general to answer your question. They're usually priced based on condition, rarity, etc. I've never seen one sell for under about $250. even 20 years ago. if it has a complete firing mechanism with all the pieces on top (body, spring, striker, etc.) that works, that piece is worth easily $100-$250. alone. Many of the guns come with the lower part of the firing mechanism only, and some were never converted to percussion firing, and still have a 0.2 inch diameter vent for friction primers. If you post a photo of it here I could assess the rarity, condition, etc. but whatever it is if you can get it for $250.-$600. or so, I'd consider it a steal. The most common maker is Sculler of New York.
If the bore's in any kind of shootable condition, that's what you want. The kind you are describing probably has a barrel cast from very good steel and if in good condition would be quite safe to shoot with proper weight and granulation of black powder, and proper projectile for the interior ballistics. These were designed to shoot a 17-pound elongated steel projectile.
The kind with trunnions at the rear as this one probably has, is hard to use as a gun, although they are perfect for use as mortars. A Coors beer can filled with plaster or concrete fits the 2.5-in. bore perfectly.
