(n) A phrase yelled by someone when his or her opponent had the upper hand in some given contest or clash. The opponent's upper hand usually results from either the yeller's mistake or the yeller's usage of too weak of an attack.
(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 Super Smash Brothers Melee, your opponent KOs you. But, you gave him a little damage by countering with your weak attack, so you yell: "Worth it!"
While playing an online game, your opponent makes a huge mistake allowing you to own him. You either type and/or yell: "Worth it!"
Usually used to describe an event where bad things happened but the end result was positive. Used as a stand alone phrase.
Friend: Dude, I you totally threw up on the roller coaster!
You: Worth it!