What is the value of a 1780 silver dollar?

Answer 1

It's not really a silver dollar. What you probably have is a Maria Theresa taler, a bullion coin that's been issued by many countries almost continuously for over 2 centuries. All of them are dated 1780 regardless of when they were actually minted.

Unless it's an original coin (very unlikely) its value is roughly the same as the current price of 1 oz. of silver. Silver prices change daily so it's best to look at a precious-metals site or the financial pages of a newspaper for current values.