A young man was introduced to Aristippus, to become his pupil.
“I shall expect ten pounds,” said the philosopher.
“Ten pounds,” said the father; “why, I could buy a slave for that!”
“Then buy one,” said Aristippus, “and you will have two slaves in your household.” (Diogenes Laertius. ii. 8, 72.)
Plato famously criticised sophists for taking money to educate young men in the art of rhetoric. Socrates never took money.