What is the value of a 1926 silver dime mark W?

Mercury dimes have the mintmark (if any) on the reverse, the "W" is the designer's initials not the mintmark. If you have an "S" mintmark you have a scarce coin worth more than melt, but it depends on the grade, a "D" mintmark is somewhat scarce but with high silver prices it is not worth much more than melt if circulated (melt is ~$2). If you have no mintmark it is an incredibly common coin worth only melt unless uncirculated.