Black would gain three points of territory. Even though, Black is the first to capture a stone, this ko is clearly favorable for White. So now we know the value of the ko, we can also look at the value per move. It takes two moves for White to capture the corner and gain 19 points. Nothing to brag about.
Many moves are worth far more than that in the middle game and early end game.
Usually, the terms are not so favorable and both sides stand to lose something by starting a ko. In this ko, both sides have eight stones at stake. If White wins the ko, he will have eight points of territory plus eight prisoners, totaling 16 points. If Black wins the ko, he will have eight plus two points of territory plus eight prisoners, a total of 18 points. Therefore, the difference is 34 points.