130.1. Main Deck cards have a Cost , listed in the upper left corner of the front face of the card. See rule 161. Rune Pools for more information.
130.2. Energy Cost
130.2.a. The numeral listed in the Cost element in the upper left corner of the card.
130.3. Power cost
130.3.a. The symbols, listed vertically, in the Cost element in the upper left corner of the card.
130.3.b. This section may not be present on every card.
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.