I'm sure that such arrangements DO work out some of the time.
What percentage are you willing to risk? If it works out 25% of the time do you want to take that crap shoot?
Maybe it is worth the effort for some guys. Just be prepared to be cheated out of your last month's payment and don't be dumb enough to pay for another month when she doesn't live up to the bargain.
I loaned an ATF some money once. I was prepared to lose the cash, but it would have been the last cash she got from me. It was about equal to one session's payment. I did get paid back, but she was a little hard to get hold of to get paid back. I didn't really pursue the issue that hard anyway. She was a "spur of the moment" type of relationship.