It would be hard to give a value without knowing details of the gun, such as maker, caliber, condition and so on reproductions range in value from less than 100 to the thousands depending on those criteria. Let me know the details and I may be of further help.
what is the value of a jukar black powder pistol 007806
10-100usd
What is the value of a 1850 colt black powder with ships engraved serial number33700
25 HUNDRED
Dikar Spain-45 Cal black powder only
$75-$100
1200
10-100 usd
50-150
$100
Depending on EXACTLY what you have, a few hundred to tens of thousands.
10-100 usd