How long has it been broken like this? By my count, at least 346 years, reckoning from the premiere of The Imaginary Invalid by Molière.
The opening scene features the protagonist iterating over a series of rationalizations for reducing his medical bills to 1/3 of the list price.
That this scene was expected to be received by an audience which could relate to the necessity of performing these sorts of arithmetic gymnastics (though perhaps exaggerated for comedic effect) suggests that this was already a well-established norm in 1673:
SCENE I.--ARGAN (_sitting at a table, adding up his apothecary's bill with counters_).
ARG. Three and two make five, and five make ten, and ten make twenty.
"Item, on the 24th, a small, insinuative clyster, preparative and gentle, to soften, moisten, and refresh the bowels of Mr. Argan."
What I like about Mr. Fleurant, my apothecary, is that his bills are always civil. "The bowels of Mr. Argan." All the same, Mr. Fleurant, it is not enough to be civil, you must also be reasonable, and not plunder sick people. Thirty sous for a clyster! I have already told you, with all due respect to you, that elsewhere you have only charged me twenty sous; and twenty sous, in the language of apothecaries, means only ten sous. Here they are, these ten sous.
"Item, on the said day, a good detergent clyster, compounded of double catholicon rhubarb, honey of roses, and other ingredients, according to the prescription, to scour, work, and clear out the bowels of Mr. Argan, thirty sons." With your leave, ten sous.
"Item, on the said day, in the evening, a julep, hepatic, soporiferous, and somniferous, intended to promote the sleep of Mr. Argan, thirty-five sous." I do not complain of that, for it made me sleep very well. Ten, fifteen, sixteen, and seventeen sous six deniers. "Item, on the 25th, a good purgative and corroborative
mixture, composed of fresh cassia with Levantine senna and other ingredients, according to the prescription of Mr. Purgon, to expel Mr. Argan's bile, four francs." You are joking, Mr. Fleurant; you must learn to be reasonable with patients; Mr. Purgon never ordered you to put four francs. Tut! put three francs, if you please. Twenty; thirty sous. [Footnote: As usual, Argan only counts half; even after he has reduced the charge.]
"Item, on the said day, a dose, anodyne and astringent, to make Mr. Argan sleep, thirty sous." Ten sous, Mr. Fleurant.
(the above is just the first half of Argan's recitations - they go on for another paragraph or two!)