Smacking your teachers ass when she bends down to help another student. Worth it!
Eating food off the male bathroom floor for 5 bucks. Worth it!
(Commonly used in video game settings.)
*The opposite can also be true, the yeller can have the upper hand and say the phrase in order to humiliate/frustrate their opponent, but this is generally looked down upon as bad sportsmanship.
While playing an online game, your opponent makes a huge mistake allowing you to own him. You either type and/or yell: "Worth it!"