Unclogging a dishwasher can cost nothing if the owner can solve the problem himself, but calling a plumber to do the job incurs at least the service call fee. As of 2015, a typical service call fee is $45 plus an hourly rate to handle the clog.
After turning off the electric circuit that powers the dishwasher, the owner should start by taking a look at the drain hose leading away from the dishwasher. It is accessible through the cabinet beneath the sink in many cases. Using a straightened-out wire hanger to pull clogs out of the hose should solve any problems there. Pulling any debris from the dishwasher drain and pouring a tablespoon of baking soda and then 2 tablespoons of vinegar down the drain should clear out any clogs there. If those steps do not work and the clog remains, it's time to call a professional.