What is the value of a 1918-w Mercury Head Dime?

Well, first off you don't have a 1918 W Mercury dime, the West Point Mint wouldn't be established for several more years and never produced any coins with a W mintmark for circulation. The W is the designer's initials, any mintmark will be on the reverse. None of the 1918 coins are rare so they are worth the price of silver, assuming they are in circulated condition, which is about $2.11 at the time of writing, if uncirculated it could be worth quite a bit more.