My furnace motor is very loud and noisy. I've been told by the repairman that it needs replacing and will be between $600-700. The furnace is only 4 1/2 years old. Is this normal for it to go bad so quickly and is that price correct?

Some inducer motors are cheap, some $400. Look on line and see what they run. They are, for the most part, very simple to replace. 4 years does seem short, mine is 12 and still runs smooth and quiet.