Why are tenor’s brains so expensive?

A woman goes into a butcher shop to get something special for dinner. She spies something which looks interesting and asks the butcher how much it costs. “Well ma’am, those are the brains of a soprano, and they’re a $1.50 a pound.” “Oh, ok”, replies the woman. “What’s that?”, she asks, pointing. “Those are the brains of a bass, and they’re $3.00 a pound.” “Oh”, says the woman, “that looks great, what is it?” “Ma’am, those are the brains of a tenor, and they are $40.00 a pound.” “That’s outrageous!”, says the woman. “Why is it so much for those brains?” “Ma’am”, replies the patient butcher, “do you have any idea how many tenors it takes to make one pound of brains?”