an arm and a leg vs pretty penny

an arm and a leg

noun
  • Usually used after the verb cost, but also often charge, pay, and spend: a very high price for an item or service; an exorbitant price 

pretty penny

noun
  • A considerable amount of money; a high price or a high income. 

How often have the words an arm and a leg and pretty penny occurred in a corpus of books? (source: Google Ngram Viewer )