Miriam T. Griffin (ed.), Seneca on Society: A Guide to De Beneficiis
(6.) 15. 2 non rei pretium sed operae solvitur (it is not the price of the thing acquired that is paid, but the price of their work). In (6.) 16. 1 Seneca says that it is their ars they sell, but at 17. 2 it is their opera again (as opposed to mental attitude). Cf. Cicero, who at Off. 1. 150 belittles those who are paid for operae, rather than artes.