Impossible to answer without knowing who made it and what it is, i.e, handgun, rifle, or shotgun.
You may have some success at the shotgun world website, (shotgunworld.com) there is a vast network of collectors and shotgun experts there that may be able to help
I believe it was known as the "Black Shadow".
Mine is on left side by site on barrow
what is the age of a .36 cal pedersoli black powder rifle serial number da89669
You must take it to a gunsmith to find out.
10-100 usd
20-300 usd
100 or so
50-100 USD
not likely anyone keeps data on re-pro black powder pistols................
10-100 usd