Rewards are indeed relative, and now there's brain research to prove it. In the figure below, researchers found that reward centers were activated depending on the context, not the size of a reward. So in a gambling task, a "win" was the best outcome, whether it was winning the most money or not losing any money under a "lose" condition.
Of course, rewards are relative under non-monetary conditions, too. We really should remember to celebrate our break-evens as well as outright victories.
Rewards are Relative and fMRI
Review- Different Types of Rewards and School Performance