The cost of a pro plumber replacing a water heater depends on the location your in.
Different states and towns have different prices and can vary between plumbers.
A new water heater though can run around $300.00 to $500.00 before including sales tax.
You then will have a service charge for the plumber to come out which is usually around $100.00 plus the labor charges.
So you could expect to pay around $800.00 to $1,000.00 for a plumber to replace the water heater.