What is the value of a 1939 Mercury dime with a w mint mark?

Please use the site's Search function to look for many other similar questions. The W is not a mint mark, it's part of the designer's monogram (A over W, for A. Weinmann).

Depending on its condition and actual mint mark, your dime might be worth $1 to $2.

Almost all coins minted before 1968 had their mint marks on the reverse side. On a Mercury dime, the mint mark - if there is one - would be located to the right of the Word ONE. If there's no mark, it came from Philadelphia; a D means Denver and an S means San Francisco. FWIW, the West Point mint didn't open until the 1980s so a Mercury dime couldn't have a W mint mark in any case.