"Oh, and don't believe wholeheartedly the post that said all originals are cheaper than modern replicas."
That's not quite exactly what the post said.....
Just for conversation sake I have a double barrel Manton that I paid about fair market of $275.00. It's all there, no chips or wood missing, locks work perfect. Bores are rough but fine for a shot gun. A new Pedersolli double will cost three times that or more. Also have a no-name single barrel "hardware store" perc. shotgun that only cost $400.00 in near perfect condition, totally shootable. A comparable gun with the same ammenities (some silver, pewter etc.) would be custom and I'm sure over $1,000.00. These are not collectable (yet!) and not very valuable.
However a fowler of Grinslade caliber from the 1770's would be worth five figures and even a lowly trade gun in shootable condition is $3,000.00 up to perhaps $7,500.00 in shootable condition. Guns like that should be insured and stored in a safe, not beat around in the woods. MHO only of course. You could have Mike Brooks build a comparable gun for ..well, less than that.
Age, condition, rarity are what seems to make one gun valuable and another not so.
Describing a gun as simply a "fowler" means almost nothing, only that it is smoothbored. Is it percussion? or flint (twice as valuable) Is it really old - pre 1800? pre 1850? Imported or domestic? Only you know the answers to those questions and only you can decide if you should shoot it or not.