You simply choose whether or not to pay that amount of as the ability resolves
If this spell was kicked, that creature gets -5/-5 until end of turn instead
An entire clans theme from Khans of Tarkir keyworded this effect as Formidable , and tons of Formidable cards still see play across formats, Temur Battle Rage being my favorite
When youre the monarch, opponents have to pay mana equal to the number of cards in their hand to attack you
Cards like Birds of Paradise, Killer Bees, and Uktabi Drake might lack in sheer power but compensate with their invaluable abilities
One of the things that has remained constant is the design on the back of Magic: The Gathering cards for obvious reasons, namely that you need to ensure that no matter what set a Magic card is from, it should be completely indistinguishable from any other card in a deck