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.
If you have a proof version (1969-S) in its Mint package it's worth about $8-9 as of 05/2011.
1970 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.