1997 Volvo 960 Smoking coming out of my car

First I am a woman, so please bear with me, for the last 3 weeks my windshield was always foggy and was very difficult to take the fog away. Two days ago there was smoking coming out of my car and the mechanic told me that it was the heater core? That my heater was not working and the smoke was really fumes coming out of my dashboard, floor, etc (very dangerous). He said that in order to repair it he needs to take the dashboard and replace the piece. It seems like a hell of a job and would like to know the price range or any other solution that is not too expensive. I don't have the amount they're asking for.