A classic red flag in the services industry.
In sex work this tends to be more of an issue for semi-pros and "friendly gals" looking to pick up some extra cash than it is for genuine mercenary hookers.
The phrase is commonly applied to any service business where there is no product that can be returned (house cleaning, tree surgery, psychotherapy, hot dog vending, midwifery, meth dealing, lap dancing, wedding singing, etc)
From a customer perspective the value of a service is worth less after it has been delivered.
Being asked to do something on the basis that you will be paid according to the customer's view of the value afterwards.
A man in a titty bar will pay $300 for a ten minute hands-off pants-up lap dance after three vodka-redbulls and an hour or two of watching T&A.
If the same dancer meets him after work instead and screws him at a motel and asks for a tip afterwards he'll notice that she has a caesarean scar and that her boobs are a little lopsided and she wouldn't let him touch her hair and her teeth are kinda yellow and besides... she liked it too. and offer her $50 bux.
(conversely, getting a customer to set a price before service and pre-pay or contract to pay that price can often benefit the service provider. People tend to inflate the value of services they hope to acquire)