A pipe burst in a doctor's house. He called a plumber. The plumber arrived, unpacked his tools, did mysterious plumber-type things for a while, and handed the doctor a bill for $600.
The doctor exclaimed, "This is ridiculous! I don't even make that much as a doctor!."
The plumber quietly answered, "Neither did I when I was a doctor." Full text

A doctor said to his car mechanic, "Your debit is several times more per hour then we get paid for medical care."
"Yeah, but you see, doc, you have always the same model, it hasn't changed since Adam; but we have to keep up to date with new models coming every year." Full text