It's just a figure of speech

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)