A new heater core itself costs between $50.00 to $200.00 depending on the make and model of the vehicle.
Then the labor to get the heater core changed out could cost from $150.00 to $500.00 depending on how hard it is to change out the heater core on your specific vehicle.
Some vehicles make the heater core more easily accessible while others require a lot more dismantling of your vehicles interior to get the heater core changed out.