Even though the 1971's are clad -- to answer your question, 40% silver halves (1965-1970) are worth $1.88 as of last Friday's silver quote of $12.72 per ounce of silver. That's with a silver content of 0.1479 ounces of pure silver.
The 1964 Kennedys and Franklins, Walkers, etc. are worth $4.60 (by my calculation) in silver content, with a silver content of 0.36169 ounces of pure silver. The silver content figures are from the 2006 Red Book Guide of United States Coins by R. S. Yeoman.
HH,
Bob
PS -- If you don't have a copy of the Red Book, buy one or check one out of the library. It's a treasure trove of coin collector information.