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Welcome. My name is ***** ***** would be glad to assist.
There are a number of factors which dictate the costs.
1. Distance to acquire the power, the shorter the best of course
2. Does the GFCI need to be a dedicated circuit?
3. Can the GFCI be wired with cable or does it need to be in conduit?
The water heater manual will have all the information on the load needed and the circuit requirement
Either way, whether a new circuit or the extension of an existing circuit requires a permit in most locals.
Ballpark could be determined from the information
I they are able to get the power from the location 20 ft away and run cable, then ballpark should run $130-$170.
The reason it is a wide range, is because the location from the contractor can add to the service call and increase the pricing especially in areas of higher traffic or longer distance etc.
Hope that helps out to set the budget,
You are most welcome, glad to assist.