Just like any other business selling its products in a free market...
You could sell at a cheap price, and hope to get a lot of customers, just like the EE conveyor belt places. As we all know having so many customers to deal with results in loss of focus and enthusiasm, giving a shit service.
You could sell at a very expensive price, and get a few very rich guys giving you thousands of pounds for a weeks work, and let your phone gather dust in between whiles.
Or you could pick a price somewhere it between to keep the number of customers to a sensible number allowing each to get a good service, repeat business, etc etc. If the chosen price creates too many customers, especially of the wrong sort, then a slight increase reduce the number. Of course the girls reputation will greatly influence the number of potential customers too!
What annoys me is that so many girls forget the value of repeat business, and building up a good customer base. The supply of punters in an area is not infinite. Give a guy a good experience and he will return, give a poor service (short time, don't kiss, dirty, etc) and he won't. So, why can't SPs understand this!!