How much will it cost to replace the water pump on a 2004 Mercedes clk320?

You can find a water pump online for $190 to $250 but it would be better to visit the dealership or a qualified mechanic. I'd go by one or two and get prices and I would be expecting to hear something in the neighborhood of $600 total. You will probably be advised to change belts at this time as they have to come off anyway.