How much does it cost to get the heater fixed

Multiply the diagnostic time for the mechanic to figure out the symptom first, since you didn't even list that, times the shop's hourly labor rate, added to the cost of the part(s), added to the time specified in the "Flat Rate Guide" to replace those parts times the hourly rate. Add the cost of other needed supplies such as antifreeze, gaskets, or sealers. We have no way of knowing even one of those items.