What is the value of a 1940 silver US nickel?

The only nickels to contain any silver were the so-called "war nickels" minted from 1942 to 1945. Your nickel, and all others ever minted except for war nickels, is made of 75% copper and 25% nickel.

Despite its age, a 1940 nickel is fairly common and is only worth a few cents above face value unless it's in uncirculated condition.
The 1940 nickel is made of the same copper/nickel blend still used today. It's not an especially rare date, worth around upwards of a dollar or so.