A new water heater costs around $400.00 to $1,000.00 depending on if it's a gas water heater or an electric water heater.
Then to have a plumber install the water heater they'll charger another $300.00 to $800.00 for labor charges and mileage to get to your house which is also known as a service charge.
When I had a natural gas water heater replaced last year though it cost me $1,000.00 for the water heater and the labor and service charges from the plumber.
If you can replace the water heater yourself then you'll only have to pay the cost of the water heater and the parts and maybe tools needed to install the water heater.