There was a similar thread a while back on this interesting topic. I'm going to set aside the tax issue as a lot of the ladies I see actually declare their income to the taxman as a legitimate profession. Granted, as in any form of self employment, there are those who will not.
As Red says this is like all business and a case of supply and demand for a product of varying quality. If I am self-employed and more people want my product at 100 than I can provide, I'm going to charge more to allow me to provide it and make more money because I don't know how long it will last. I'll test out 120, 150 carefully until it balances but also consider where I think the top end is to not make my regular business go away. In this period, if someone fancies my product they pay this price. If I push too hard, I'll come back down a bit to where the balance is. If I want to temporarily increase business, I may put special offers on. I also may do some long term deals for customers who pay me a lot every month. It's the way it is and how I'd run my one man business with a certain capacity to maximise profit. Reality. Petrol, consulting, supermarkets, builders etc
Then there's complexity and emotion thrown in with questions from our side like, 'how much do I feel is greedy to have sex with me when I can't pay those prices?'. Also, some SP's who like to give little in return or deceive for short term gain.
In the end, the market decides (us) based on quaility, supply, ability to pay against service and we shop around. There'll be a top end, a good middle and a low end, with tax payers and avoiders, professionals and rip off artists in the mix. And the average rate for each category is dictated by demand.