Farmers aren't the only ones with hoes

Just a quick one before I head to work:

I was driving past my place of employment the other day in an intellectual fog, and skewed as my perspectives on everything were at the moment, I got to thinking (a dangerous habit, I know): I'm being rented.

(For those of you who don't know me, and that's likely all of you, I work at my local county library as a "Student Shelver") I'm paid by the hour. There is no commission for shelving books, and I am not rated on how well I do my job - simply that at some point, I do it. I am literally renting myself out to the county.

...Which leads me to wonder: how does a prostitute determine the going rate? I suppose we have to account for the presence of competition in the area, whether she works for a pimp or freelance, and based on skill. But, the question is: would the charge be incurred for each service rendered, or since we know time is money, by the amount of time it takes? Is someone actually paying for the sex, or the time it takes from her busy schedule?
I assume it must be some combination.
But if a pimp is involved, from what I've gathered from movies (a surprisingly reliable source if you're willing to filter certain theatric elements ), he collects much of the revenue for himself. In which case, does he pay his ho by the hour, or does she earn commission?