A doctor notices a sidewalk stand that says 'brain

A doctor notices a sidewalk stand that says 'brains for sale.' He goes over to investigate and sees a sign that says 'Doctor brains $8.00 a pound’ and another sign that says ‘Paramedic brains $12.00 a pound, Nurses brains $30.00 a pound, truck driver $40.00 a pound and lawyers brains $90.00 a pound.’
So he asks the man behind the cashregister, “how come his brains are only worth 8.00 and a lawyer's worth 90.00?”
The man replies, “do you know how many lawyers it takes to make a pound of brains?”