Depends on the caliber. A .45 caliber pistol uses about 40 grains of black powder. Contact the maker's website and download the owner's manual for YOUR gun.
How much is a old black powder only 45 cal long barrel pistol worth?
Follow the recommendations in the owner's manual.
First, find a copy of the owner's manual for YOUR pistol. Second, use ONLY black powder, or a modern black powder substitute, such as Pyrodex. The ROUGH rule of thumb is one half the bore diameter in grains of powder. A .32 cal would use ABOUT 12-16 grains of powder.
Chances are that if you overload the charge in a revolver style black powder pistol, you can blow up the cylinder or blow the barrel off the end of the gun.
40 grains of fffG
10-100 usd
Assuming you have a single shot ball and cap black powder pistol, anywhere from $50 to $100. These are modern manufacture reproductions, and relatively cheap.
10680
Depending condition, if gold inlay engraving, about 700 to 1000..
$100-$250. depending on specifics and condition.
50-300 usd
There were thousands of black powder guns made in Spain in any given year. These range from very cheap to well made. You did not give us much information to work on- whether it is a rifle, shotgun, pistol, or cannon.