What is the value of a 1999 us quarter?

If you found it in change, 25 cents unless it's one of the rotated-die errors that occurred when the first Delaware quarters were minted; see the Related Question for more.

A nice uncirculated one might sell for a couple of dollars, while a top-quality coin could bring as much as $25-30, as of 04/2010.

A 1999 clad proof retails for $5-10, and a silver "Prestige" proof goes for $13-25.