What is the value of a 1965 Kennedy half dollar?

From 1965 to 1969 US half dollars were struck in "debased" silver. The outer layer is 80% silver bonded to a core that's mostly copper, for an overall content of about 40%.

Unless the coin is in uncirculated condition or is from a so-called Special Mint Set, it's basically worth its metal content, about $7 to $8 as of 05/2011.

Note that from 1965 to 1967, US Coins didn't have mint marks and proof sets weren't made so there are no 1965 proofs, or D or S mint coins.

Other dates1970 halves were also made of 40% silver but were only sold in special collectors' sets.

All circulation-strike halves dated 1971 and later don't contain any silver at all. They're made of copper-nickel like dimes and quarters. Any that you find in change are only worth 50 cents, regardless of many myths and urban legends to the contrary.