The Cost of Experience: Furnace repair bill

I saw this a while back, and thought that it raises a good point: the cost of experience! 

A furnace repairman coming to a home and after looking at the furnace for about a minute and a half, listening to the rumbles and gurgles. He takes his hammer out and at once precise place he hits the furnace. The furnace starts up and runs fine as if it was brand new.

The bill was $200.

The homeowner asks why so much when all he did was hit it once with a hammer?

The repairman takes back the bill, and itemizes the bill still totaling $200.

                      Cost of hammering, $1
                      Knowing where to Hammer $199