Socrates narrates a dialogue between himself and Callias about sophists – that is, those who (unlike Socrates) do get paid to teach. He emphasizes that they are usually foreigners.
20a-c. And as for that, there is another man here, from Paros, a wise man, who I perceived was in town; for I happened to meet a man who has paid more money to sophists than all the others, Callias, the son of Hipponicus. So I questioned him (for he has two sons):
“Callias,” I said, “If your two sons had been born colts or calves, we would have been able to get and hire an overseer for them who could make the two of them noble and good in their appropriate virtue, and he would have been someone from among those skilled with horses or skilled in farming. But as it is, since they are two human beings, whom do you have in mind to get as an overseer for the two of them? Who is knowledgeable in such virtue, that of human being and citizen? For I suppose you have considered it, since you possess sons. Is there someone,” I said, “or not?”
“Quite so,” he said.
“Who,” I said, “and where is he from, and for how much does he teach?”
“Evenus,” he said, “Socrates, from Paros: five minae.” And I regarded Evenus as blessed if he should truly have this art and teaches at such a modest rate. As for myself, I would be pluming and priding myself on it if I had knowledge of these things. But I do not have knowledge of them, men of Athens.
Callias is said to have spent more money than anyone else on sophists. If price is all that matters, he should have found the best sophist.