an arm and a leg vs at a premium

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 

at a premium

prep
  • At a high, or higher than expected, price. 

  • Scarce, in short supply. 

  • Above par value. 

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