I'm 5 years out of college and I think my position has changed significantly since then. Until probably a year out of college I only thought very abstractly about questions like this and believed there was a large class of problems for which there is no price.
Until a year or so after I graduated, I think I believed that most people (hookers excluded) wouldn't offer sex at any cash price. I always thought those transactions generally HAD to occur masked as "I'll buy you some beers/some dinners/we'll date/any time or cash value exchange in return for sex"
My more recent experience with regards to your experiment is almost the exact opposite of your findings. I'm consistently shocked by how LOW a price people are willing accept for all manner of tasks, including sex.
Not to sound way too shady, but I've since found that the cash price point for sex for most women is not only "no price", but in fact far lower than I would have imagined. Or maybe I've just met way sluttier girls since graduating. The price for most men is basically zero.
I'll also say there's likely some sort of phase transition here where the answer to "what kind of girl she is" is dependent on some socioeconomic indicator. Hypothesis: Money and status buy people a LOT of self importance and stricter adherence to a moral code. People don't have to bend their espoused morals unless faced with tough questions.
Re your experiment: - saying and doing are different, especially since the question seems to have been explicitly phrased as an exchange of sex for money. there are all sorts of ways to phrase the question where sex is understood but without actually saying it. - £1MM is doesn't even feel like a real number to most students. hypothetical questions get hypothetical answers.
Like @Confusion, I now generally believe there's a price for everything. If it can be measured in dollars, thats an easy question to answer. The hard questions are those that are measured in things money can't buy.