am arm and leg

In George Washington's days, there were no cameras. One's
image was either sculpted or painted. Some paintings of
George Washington showed him standing behind a desk with
one arm behind his back while others showed both legs and
both arms. Prices charged by painters were not based on
how many people were to be painted, but by how many limbs
were to be painted. Arms and legs are 'limbs,' therefore
painting them would cost the buyer more. Hence the
expression, 'Okay, but it'll cost you an arm and a leg.'
(Artists know hands and arms are more difficult to paint.)