What is the value of an 1800 US silver dollar?

$750 retail if well-worn, about $4500 if almost uncirculated.

If you actually have such a coin you'll need to have it appraised to make sure it isn't a copy or outright counterfeit, because there are many such pieces. The Far East is a particularly notorious source of counterfeit dollars. The quality ranges from laughably bad to almost undetectably good.