What is the value of a 1972D Dime that has one side heads and the other is blank?

This could happen if two planchets got into the die together (there will be a matching error with the tails side but no heads). It would have to be examined by an expert to verify that it is an actual mint error. That will cost you about $40 for an official certification. I don't deal with error coins often, but would estimate that such a coin would bring $50 or so if certified.