How much is a 1991 silver penny worth in mint condition?

The U.S. never minted a silver penny. It would have more than a dime's worth of silver in it.

1983 and later cents are all made of copper-coated steel so you may have a cent where the copper coating was removed with acid (worth only a penny) or was never plated in the first place (worth a significant amount). It would need to be inspected by a dealer who handles error coins.