Well, she’s advertised as a pretty woman, not a smart one.
And now, with a little less jest: It’s a stable income over a week, with a guy she evidently likes to at least look at. No need for advertising in that time. Meals are paid, as is time off. Also, the 21$ assume that she is paid 24 hours a day (3000$ ÷ 24 ÷ 6 = 20,83333…$) so she gets paid for sleeping, eating, essentially her own time. She also gets the opportunity to scout new, well off clients.