What is the value of a 1967 US half dollar with an upside-down eagle?

The first thing you should do is compare it with other coins in your pocket change (you did do that, right?) ALL U.S. coins minted since the 19th century have the front and back oriented 180º to each other so that the sides point oppositely when a coin is flipped side to side like the page of a book. If that's true of your coin, it's perfectly normal. If not, you should have it examined in person by a dealer or other expert to determine if it's genuine.