A surgeon calls a plumber to fix his leaky faucet
A surgeon calls a plumber to fix his leaky faucet. The problem requires an easy fix and the entire job takes less than two minutes. Before leaving, the plumber says, "That will be $200." The surgeon replies, "I am a surgeon and even I do not charge $100 a minute." The plumber says, "Yeah, I didn't either when I was a surgeon. Why do you think I switched?"
This is a plumber joke - the joke indicates that plumbers charge ridiculous amounts of money for their services, so it might be considered insulting toward plumbers, however the punchline indicates that the plumber used to be a surgeon, so apparently the plumber found it to be a better job than being a surgeon. In light of this, the joke is probably not an insult directed at plumbers.
The humor in the joke comes from the surprise that the plumber used to be a surgeon before he was a plumber. Most people wouldn't consider a job that takes less than 4 years to become fully qualified for and does not require a degree to be better than being a surgeon, but if you can charge so much for so little effort, perhaps it would be worth it.