How much is a musket ball worth?

Unknown, but it depends on how old it is and where it came from. Musket balls are still being made today for black powder enthusiasts, and they make good slingshot ammo too. These would be worth no more than the current value of lead. A musket ball from an important battle, duel, or other historical event would surely have monetary value, but it would depend on how much the buyer is willing to pay, and proving it is genuine will be difficult. A musket ball is much easier to counterfeit than a coin.