From 1965 to 1967 no mint marks were put on any U.S. coins. They weren't restored until 1968 when the mints were able to catch up with demand for the then-new clad coinage.
Please check again and determine what you have, then post a new question. If the coin actually has an S mint mark it's been altered, so it would only be worth its silver content value, about $2.50 as of 11/2008.
The 1966 Kennedy half dollar is 40% silver and worth about $5.00.
A 1941 US half dollar in good condition is worth about $9. If it is in uncirculated condition, it would be worth about $26 to $40.
A 1860 Liberty Seated Half Dollar in good condition (G4) is worth: $250.00.
In average condition a 1936 Walking Liberty half dollar is valued at $18.00.
This half dollar is worth between seven and three-hundred dollars, depending on the condition.
The 1966 Kennedy half dollar is 40% silver and worth about $5.00.
It depends on the year and condition, but it's worth at least half a dollar.
A 1941 US half dollar in good condition is worth about $9. If it is in uncirculated condition, it would be worth about $26 to $40.
the 1972 Kennedy half dollar is worth 50 cents in circulated condition, or about a dollar in uncirculated condition
About $2.00
A 1860 Liberty Seated Half Dollar in good condition (G4) is worth: $250.00.
It's currently worth about $4 for its silver content.
As of February 2018, it's worth about $2.50 for the silver.
The value of a US 1964 half dollar in circulated condition is around $10. in uncirculated condition they are worth around $16.
As of 8 April 2014, one is worth $6 for the silver. If it's in uncirculated condition, it could go for $8 or $9.
In average condition a 1936 Walking Liberty half dollar is valued at $18.00.
1962D Franklin Half Dollar: in very fine condition is worth $3.00, uncirculated is worth $7.00.