In 1971 the us mint started using copper cores and nickel jackets around them. It looked silver when new, but is all a slug of clad metal. It is copper in the center, so that grows in value now. The half dollar was changed from Ben Franklin to John F. Kennedy. It was done after his death. The coin is now struck at the mint in proof style for collectors single or in birth year sets. No general circulation. I got one in my change, gas stations and c stores still use them as money.
They minted 302,097,000 of the Denver mint mark. The value for resale is 75 cents a coin, in average or less condition. Near uncirculated or proof you go dollar fifty to five dollars. The value of the coin will never be that great. But the fun of finding one in your change will not diminish. With over a quarter of a billion out there, you are bound to find some if you look long and hard and do lots of change. Ask the cashier if they have any half bucks they want to chuck at you.
The fifty cent piece may change someday, coins often do. But given the cult status of JFK, this one may not for a long time.