What is the value of a 1999 Sacagawea gold dollar coin?

No such coin like this exists. In 1999 the only small dollar produced by the US was the Susan B. Anthony dollar which was the same colour and compositon as the quarter. The Sacajawea dollar would first be struck dated 2000. If you indeed have a 1999 Susan B. Anthony coin, it is only worth face value unless in mint packaging.