In the past, when there were no cameras, images were either sculpted or painted.  Prices charged by painters were not based on how many people were painted, but by how many limbs were to be painted.  Arms and legs are “limbs” therefore painting them would cost the buyer more.  This is where the expression “it’ll cost you an arm and a leg” comes from.  We use this expression nowadays when something is very expensive. (ex. “My phone bill costs an arm and a leg!”)

Read about some more interesting origins: http://all-that-is-interesting.com/post/5692281236/interesting-origins-of-7-common-english-idioms