As you probably know, Bob, JFK half dollars were issued in 90% silver in 1964. Then, from 1965 to 1970, these coins contain 40% silver. After that, today's half dollars contain no silver at all, so all JFK halves minted after 1970 are worth face value, that is, one-half US dollar. Bicentennial JFK half dollars, dated 1776-1978 with Independence Hall on the reverse side, contain no silver and are worth face value. If you have an absolutely uncirculated example, a collector might pay a few US dollars for it.
Pre-1971 coins are worth their silver content. Here is the amount of pure silver in the pre-1971 coins:
1964: 0.3617 troy ounces
1965-1970: 0.1479 troy ounces
Silver price at the time of this writing is around $13 US dollars per troy ounce. That makes the value of these coins:
1964: 0.3617 x $13 = $4.70
1965-1970: 0.1479 x $13 = $1.90
The value of silver changes every day. Look up the current value at kitco.com, then do the multiplication to see what they are worth today.