Depending on the size of the water heater and the type of the water heater a new water heater ranges in cost from $300.00 to $500.00 and in some cases a water heater can cost as high as $1,000.00
To have a plumber replace the water heater it can cost $100.00 for the service call plus another $200.00 to $300.00 for installation costs.
I would call around local plumbing companies and get some estimates on what they would charge to replace the water heater.