The genitive is used with verbs signifying to buy, sell, cost, value, exchange. The price for which one gives or does anything stands in the genitive.
ἀργυρίου πρίασθαι ἢ ἀποδόσθαι ἵππον
to buy or sell a horse for money R. 333b, Θεμιστοκλέᾱ τῶν μεγίστων δωρεῶν ἠξίωσαν
they deemed Themistocles worthy of the greatest gifts 4.154, οὐκ ἀνταλλακτέον μοι τὴν φιλοτῑμίᾱν οὐδενὸς κέρδους
I must not barter my public spirit for any price 19.223. So with τάττειν rate, μισθοῦν let, μισθοῦσθαι hire, ἐργάζεσθαι work, and with any verb of doing anything for a wage, as οἱ τῆς παρ᾽ ἡμέρᾱν χάριτος τὰ μέγιστα τῆς πόλεως ἀπολωλεκότες
those who have ruined the highest interests of the State to purchase ephemeral popularity 8.70, πόσου διδάσκει; πέντε μνῶν
for how much does he teach? for five minae A. 20b, οἱ Χαλδαῖοι μισθοῦ στρατεύονται
the Chaldaeans serve for pay C. 3.2.7.
a. The instrumental dative is also used. With verbs of exchanging, ἀντί is usual (1683).