What is the value of a 1942 W Mercury head dime?

That's not a mint mark. It's the monogram of the designer A.A. Weinmann. The West Point Mint did not open until the 1980s and only strikes commemorative coins and cents.

Mercury dimes had their mint mark in the space to the right of the Word ONE on the dime's back. For a 1942 dime it will either be blank (Phila.) or have a small "S" for San Francisco, or a "D" for Denver.