It seems paying for sex may have evolutionary roots (pun unintended).
… found that male macaques pay for intercourse by using grooming as a currency.
The article reports that:
On average, females had sex 1.5 times per hour, but immediately after being groomed by a male partner, this rate jumped to 3.5 times per hour. After grooming, the female was also less likely to offer herself to males other than her grooming partner.
It seems market forces also come into play, as the male only has to groom for 8 minutes if there plenty of females around (as compared to about 16 minutes if there aren’t other females around). Obviously you have to try harder when the prospects are fewer (I bet there are plenty of men out there who’ve experienced that) 🙂