This happened to someone I know recently. They had a great hour punt with a hot girl, they seemed to be getting on well so he asked her out for a drink. They were out for a couple of hours, he bought her drink, a meal and drove her home after a great time. Upon exiting the car she said I had a great time, thanks, that'll be another £400 quid!!!! The whole 3 hour punt (45 mins of sex) cost him £550, that's not including all her drinks, a meal in a flash restaurant and driving her to and from the city (about 50 miles).
Like Smiths i'd only do I off the clock, but occasions like that will probably be few and far between. I've never asked as I don't want to make things awkward for future punts, just would be nice to know some of the girls a bit better as the sex would probably be a lot better