If they are 90% There are 0.3617 troy ounces of silver in 2 silver Washington quarter(s). » Each Washington quarter contains 0.1808 troy ounces of silver and is valued at $3.33 when silver is at $18.42 / oz.
Coinflation has them at $3.3312. It is a great website for getting bullion value. http://www.coinflation.com/