at a premium vs pretty penny

at a premium

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

  • Scarce, in short supply. 

  • Above par value. 

pretty penny

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

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