IDIOMS

not worth a brass farthing

(idiomatic) Worth nothing or next to nothing.

Example: 1915, Sax Rohmer The Yellow Claw, ch. 24,
  One's own private knowledge of a man's character is not worth a brass farthing as legal evidence.
2005, Jeremy Page, "Russia is now ripe for freedom revolution, warns Solzhenitsyn," timesonline.com (UK), 7 Jun. (retrieved 29 Sep. 2008),
  Democracy is not worth a brass farthing if it is being installed by bayonets.