Pour des prunes

Une prune is a plum - not to be confused with un pruneau, a prune.  While this lovely fruit is fully appreciated for its nutrition properties nowadays, it has also long been associated with something of little or no value in French.  If you do something "for plums," it means that something you put some effort into produced no valuable results.  It is equivalent to the English for nothing, for peanuts.

The expression may be used by itself, but it is more often heard with verbs such as:
*travailler pour des prunes - work for peanuts 
*compter pour des prunes - count for nothing
*faire quelque chose pour des prunes - do something with no result, uselessly

Which begs the question: why so little consideration for such a generous fruit, bursting with vitamins and antioxidants?

Origin

To understand, we have to go back to the 12th century, at the time of the second crusades to the Holy Land.  On July 24, 1148, the crusaders lay siege to Damascus.  In addition to its strategic position, the city is rich and particularly famous for its plums, the flavor of which is said to be exceptional.

But the conquest is cut short. Damascus will not give in.  After heavy losses on both sides, the crusaders are in poor shape and decide to turn around. The disastrous siege only lasted four days.

Rather than coming home empty-handed, the Crusaders bring back some plum trees from Damascus.  A meager fortune for all the time and lives lost in battle.  Legend has it that the French king was furious about the defeat and would have exclaimed: "You only went there for plums!"  And that's how plums acquired their bad reputation.  Small consolation or fruit revolution? Everyone can choose the moral of this story.

Examples

On a travaillé toute la journée pour des prunes; on n'a rien accompli.
We worked all day long for nothing; we didn't accomplish anything.

Son avis compte pour des prunes; personne ne l'écoute.
His opinion doesn't count; no one ever listens to him.

J'en ai assez de travailler pour des prunes.  Je vais chercher un travail mieux rémunéré.
I've had enough of working for peanuts. I'll look for a better paying job.