The Mint changed the composition of Kennedy Half Dollars this year. Instead of having some silver they switched over to a clad composition. The 1971-D Kennedy Half Dollar is really common with hundreds of millions of examples produced. More than likely, most of them still survive and are probably sitting in banks, vaults or even in jars in people's homes being kept as conversation pieces.
Most however do not command a premium. Only in MS67 do they begin to command somewhat of a premium as there are probably less than 1,000 examples known. In MS68, they are very difficult to come by and more than likely there are less than a dozen known with none existing in high condition.