Laudator Temporis Acti

Aristippus of Cyrene, in Diogenes the Cynic, Sayings and Anecdotes with Other Popular Moralists. Translated with an Introduction and Notes by Robin Hard (Oxford: Oxford University Press, 2012), p. 127, with note on p. 245:

He said that teachers deserve to receive handsome fees from their pupils, from the gifted ones because they learn so much, and from the incompetent ones because they cause so much bother.*

(Excerpts from the Manuscripts of Florilegia of John Damascene, 2.13, 145; G6)

so much bother: the Gnomologium Vaticanum ascribes very similar sayings to both Aristotle (57) and Isocrates (355), and this was doubtless a commonplace that came to be attached to a variety of suitable people.

The Greek, from Gabriele Giannantoni, ed., Socraticorum Reliquiae, Vol. I (Naples: Bibliopolis, 1983), p. 187, no. 6:

ὁ αὐτὸς [scil. Aristippus?] ἔλεγε "μεγάλους δεῖ λαμβάνειν μισθοὺς μαθητῶν τοὺς διδασκάλους, παρὰ μὲν τῶν εὐφυῶν ὅτι πολλὰ μανθάνουσι, παρὰ δὲ τῶν ἀφυῶν ὅτι πολὺν κόπον παρέχουσιν."

Leo Sternbach, ed., Gnomologium Vaticanum e Codice Vaticano Graeco 743 (Berlin: W. de Gruyter, 1963), p. 27, no. 57:

Ὁ αὐτὸϲ ἔφη· μεγάλουϲ χρὴ λαμβάνειν μιϲθοὺϲ παρὰ μὲν τῶν εὐφυῶν, ὅτι πολλὰ ὠφελοῦνται, παρὰ δὲ τῶν ἀφυῶν, ὅτι πολλὰ πράγματα μανθάνοντεϲ παρέχουϲι τοῖϲ διδάϲκουϲιν.

Sternbach, p. 136, no. 355, isn't visible on Google Books, but I think the Greek reads as follows:

Ἰϲοκράτηϲ ἔλεγε· μεγάλουϲ δεῖ λαμβάνειν μιϲθοὺϲ παρὰ τῶν μαθητῶν τοὺϲ διδαϲκάλουϲ, παρὰ μὲν τῶν εὐφυῶν, ὅτι πολλὰ μανθάνουϲι, παρὰ δὲ τῶν ἀφυῶν, ὅτι πολὺν κόπον παρέχουϲιν.

Albert Anker, Die Dorfschule
Thanks to Ian Jackson for a photocopy of the page from Socraticorum Reliquiae.