To do what? Humiliate yourself on national television? Murder a child?
I would say that I definitely have a price on the former (If it helps me retire comfortably 10 years earlier, of course I'd do it), and do not on the latter. I suspect most people feel the same way -- it depends on what you ask them to do.
It would depend on the situation and the person. Money is not all there is for 'price', as has been mentioned. If the price is friends and family being hurt that makes it much murkier indeed.. what if it is a person that 'has nothing to lose' but must do something or innocent people are killed... I think there's always some level to lose but it would depend on the skill of the manipulator as to how the game went down.
"It is not length of life, but depth of life." ~ Ralph Waldo Emerson