For what amount of money would you have sex with a stranger?
The scenario is an average person, not especially beautiful or ugly, of your preferred sex, that is in no special relation to you, your friends, family or workplace, basically a standard stranger that would not be special enough to be considered as a partner, comes on to you and offers you money for spending a night with him/her.
You KNOW you'll be safe, there will be no consequences from this, he(she) is willing to use contraception, and has no other special desires except standard vaginal intercourse(/what's applicable for homosexuals), and keep completely discrete about this...no one will ever know what you did.
The money is cash, right there, clean...for what amount of money would you have sex with this person?
Or, even better: HAVE you already had a person pay you for having sex with them?(Casual prostitution among friends...)