Rule 204.3.b

When Costs within Instructions appear in spells, activated abilities, or in later parts of triggered abilities, the cost must be paid to get the effect during resolution of the spell or ability. Example: Bullet Time is a spell that reads “Pay any amount of [A] to deal that much damage to all enemy units at a battlefield.” As the spell resolves, its controller pays any amount of [A] as a cost, and deals that much damage to all units at a battlefield as the effect. Example: Ancient Henge is a gear that reads “[Reaction][>] [E]: Pay any amount of Energy to Add that much [A].” As the ability resolves, its controller pays any amount of Energy as a cost, and Adds that much [A] as the effect. Example: Insightful Investigator is a unit that reads “When you play me, choose an opponent. They reveal their hand. You may pay 2 XP to choose a card from their hand. If you do, they discard that card and draw 1.” As the ability resolves, its controller may pay 2 XP as a cost, and chooses a card from that player’s hand as the effect.