What is the value of a 1799 us silver dollar?

If it's genuine, it could retail for anywhere from $1000 to over $300,000 depending on its condition and variety.

But before you pay off your mortgage, the market has been flooded with fake dollars and dollar-size coins over the last couple of decades to the point that there are more counterfeits than genuine coins. In addition there are legitimate "replica" dollars that have the Word "COPY" somewhere in small letters, as required by law. Counterfeits of course won't, LOL! And finally, there are modern 1 oz. silver bullion pieces that carry the fake date 1799; these are worth roughly whatever the current price of silver is.

Bottom line, you'd need to have the coin authenticated by someone who deals with early U.S. coinage.

If you have a small coin slightly larger than a quarter, gold in color, with a portrait of a U.S. president on the front it's not gold OR silver, and it's not from 1799 - it's one of the new presidential dollars and it's made of brass.