What is the cost of a Ford Taurus Heater core in NY state?

Ford Taurus heater has been having probs in the winter. It gets below zero and it blows warm air, then cold air. Does it pay to just replace the heater core? If the heater core is replaced will the heat work better, or should I not bother? What is the cost of a Ford Heater Core for NY state? How many hours does it take to replace the heater core?|||replacement cost are higher than the core it self 400-600 hundred and if its not leaking leave alone .