The cost to replace a water pump on a car depends on what type of car you have. Some cars the water pump is cheaper and easier to get too and others are a pain in the butt to get too and can take hours.
Some cars you have to remove a lot of crap to get to the water pump and others the water pump is very easily accessible.
Some shops charge $50.00 per hour and others charge $100.00 per hour so you can be looking at $400.00 for the low end to get the water pump changed depending on how many hours it takes to replace the water pump.
The water pump itself isn't that expensive. Some water pumps for cars are only around $30.00 it's the cost of the labor that will empty your wallet.