130.4. Effects that need to determine a card’s cost for any purpose always use its base cost, even if that cost is altered or ignored as the card is played. Example: Sky Splitter is a spell that costs 8 Energy and says in part “This spell's Energy cost is reduced by the highest Might among units you control.” Lux, Illuminated is a unit that says “When you play a spell that costs 5 or more, give me +3 [M] this turn.” If a player who controls Lux plays Sky Splitter, Lux’s ability will trigger, regardless of how much was actually paid for Sky Splitter.