Die Young

If the target creature becomes an illegal target, Die Young doesn't resolve and none of its effects happen. You won't get {E}{E} or be able to pay any {E}.

You choose the target creature as you cast Die Young. You don't choose how much {E} to pay until Die Young is resolving. No player may take actions between the time you choose how much {E} to pay and the time the creature gets -1/-1 for each {E} paid this way.

Some triggered abilities state that you "may pay" a certain amount of {E}. You can't pay that amount multiple times to multiply the effect. You simply choose whether or not to pay that amount of {E} as the ability resolves.