Cost estimate to remove and replace heater core?

What is your question?

No heat at operating temp, blower and heater box doors/deflectors working normally, cooling system has been flushed twice, was indicated to be thick and way past due, suggested heater core was probably clogged and could not be completely flushed...no leaks inside or out

What makes this problem better or worse?

Some heat when driving, no heat when ideling