The Cake Is A Lie

  Roughly translates to "your promised reward is merely a fictitious motivator". 

Implies that a promised reward for one's work is false, with the one proffering the reward never intending to give it in the first  place, possibly because the reward never even existed.

This phrase can be used to express grief or frustration regarding any situation where there is an imbalance between effort and reward. 

This phrase was popularized by the game  "Portal" by Valve Software Corporation.