"Brainshopping" joke

A man is passing a butcher's shop and sees a sign: "Special Offer - Brains"
Cow brainsa penny a pound
Sheep brains2 pounds a pound
Pig brains2 pounds a pound
Doctor brains50 pounds a pound
Engineer brains50 pounds a pound
Programmer brains50 pounds a pound
Lawyer brains1000 pounds a pound
and he goes into the shop and says to the butcher "Excuse me, I couldn't help noticing your sign - I suppose the cow brains are so much cheaper than the other brains because of the Mad Cow Disease scare."
"That's right" says the butcher.
"And," continues the man, "I suppose the lawyer brains are so much more expensive than the other brains because they're such high quality."
"Not at all," says the butcher "do you know how many lawyers you gotta kill to get a pound of brain?"