Adam Clarke on Numbers 22:7

The rewards of divination - Whoever went to consult a prophet took with him a present, as it was on such gratuitous offerings the prophets lived; but here more than a mere present is intended, perhaps every thing necessary to provide materials for the incantation. The drugs, etc., used on such occasions were often very expensive. It appears that Balaam was very covetous, and that he loved the wages of unrighteousness, and probably lived by it; see 2Pet 2:15.