Politician's brain

There was only one cure for his problem...a brain transplant. Having decided
to have the operation privately he was given the choice of what sort of brain he
should receive.
"The brain of a top-notch mathematician will cost you $10,000. That of a
world-renowned astro-physicist will set you back $20,000. But if you want the
brain of a politician it will cost $40,000!"
"Why on earth does the brain of a politician cost so much more than those of
people who are clearly much cleverer?" he asked, very much surprised.
"Ah well, you see, the politician's brain has hardly ever been used."