on offer vs unobtainable

on offer

prep
  • For sale at a reduced price. 

  • Available to take; at hand; being offered or to be offered. 

unobtainable

noun
  • Someone or something that cannot be obtained. 

adj
  • Unable to be obtained: not able to be acquired or reached. 

How often have the words on offer and unobtainable occurred in a corpus of books? (source: Google Ngram Viewer )