The guinea is the ordinary unit for a professional fee
Depends on the profession, Doc. When I was a student in Liverpool, aeons ago, the professional lasses on Lime Street would settle for ten bob* (Understand, this is only hearsay. I never engaged the young ladies’ services - their appearance was a convincing argument for chastity). On second thoughts, it depends on the social stratum, too. I remember [correction: I think I remember] an episode from Halliday Sutherland’s The Arches of the Years (1932) where a top-drawer young lady submits a quotation: “The fee is five guineas”. So I suppose you and the OED are right, after all. As usual.
*(Half a guinea less a tanner, if we want to be pedantic)