I have heard it attributed to both Churchill and George Bernard Shaw but whoever the story is apposite:
On a train a man asked a pretty girl if she would allow him a kiss if he paid her ?1m ~ She smiled and said "Yes"
The man then asked "If I gave you ?5 would you sleep with me?"
"Certainly not!" exclaimed the girl "What sort of girl do you take me for?"
"Madam" the man replied "We have already established the facts ~ we are now merely negotiating the price."