What is the value of a Lady head dime?

It's not a "lady head" dime, that's a picture of Miss Liberty, so it's sometimes called a Liberty head dime. However, the Liberty cap she is wearing looks a lot like the helmet worn by the god Mercury so these coins are almost universally known as Mercury dimes.

They were issued for over 30 years at 3 mints, so you'll need to post a more specific question with the coin's date and whether there is a letter D or S next to the Word ONE on the back.