If you are at an all$-$you$-$can$-$eat buffet dinner and you are considering whether to eat another dessert, you, as a rational consumer should decide to eat the desert

if it brings you more pleasure than the price you paid for the meal.

if it brings you as much pleasure as the desert costs if it is ordered a la carte.

if it brings you any pleasure at all

no matter what because you want to get more for your money.