From the Comprehensive Rules (Khans of Tarkir)
- 117. Costs
- 117.1. A cost is an action or payment necessary to take another action or to stop another action from taking place. To pay a cost, a player carries out the instructions specified by the spell, ability, or effect that contains that cost.
- 117.2. If a cost includes a mana payment, the player paying the cost has a chance to activate mana abilities. Paying the cost to cast a spell or activate an activated ability follows the steps in rules 601.2e–g.
- 117.3. A player can't pay a cost unless he or she has the necessary resources to pay it fully. For example, a player with only 1 life can't pay a cost of 2 life, and a permanent that's already tapped can't be tapped to pay a cost. See rule 202, "Mana Cost and Color," and rule 602, "Activating Activated Abilities."
- 117.3a Paying mana is done by removing the indicated mana from a player's mana pool. (Players can always pay 0 mana.) If excess mana remains in that player's mana pool after making that payment, the player announces what mana is still there.
- 117.3b Paying life is done by subtracting the indicated amount of life from a player's life total. (Players can always pay 0 life.)
- 117.3c Activating mana abilities is not mandatory, even if paying a cost is.
Example: A player controls Lodestone Golem, which says "Nonartifact spells cost more to cast." Another player removes the last time counter from a suspended sorcery card. That player must cast that spell if able, but doing so costs . The player is forced to spend if enough mana is in his or her mana pool, but the player isn't forced to activate a mana ability to produce that . If he or she doesn't, the card simply remains exiled.
- 117.4. Some costs include an or an X. See rule 107.3.
- 117.5. Some costs are represented by , or are reduced to . The action necessary for a player to pay such a cost is the player's acknowledgment that he or she is paying it. Even though such a cost requires no resources, it's not automatically paid.
- 117.5a A spell whose mana cost is must still be cast the same way as one with a cost greater than zero; it won't cast itself automatically. The same is true for an activated ability whose cost is .
- 117.6. Some mana costs contain no mana symbols. This represents an unpayable cost. An ability can also have an unpayable cost if its cost is based on the mana cost of an object with no mana cost. Attempting to cast a spell or activate an ability that has an unpayable cost is a legal action. However, attempting to pay an unpayable cost is an illegal action.
- 117.6a If an unpayable cost is increased by an effect or an additional cost is imposed, the cost is still unpayable. If an alternative cost is applied to an unpayable cost, including an effect that allows a player to cast a spell without paying its mana cost, the alternative cost may be paid.
- 117.7. What a player actually needs to do to pay a cost may be changed or reduced by effects. If the mana component of a cost is reduced to nothing by cost reduction effects, it's considered to be . Paying a cost changed or reduced by an effect counts as paying the original cost.
- 117.7a If a cost is reduced by an amount of colored mana, but its colored mana component doesn't contain mana of that color, the cost is reduced by that amount of generic mana.
- 117.7b If a cost is reduced by an amount of colored mana that exceeds its mana component of that color, the cost's mana component of that color is reduced to nothing and the cost's generic mana component is reduced by the difference.
- 117.7c If a cost is reduced by an amount of mana represented by a hybrid mana symbol, the player paying that cost chooses one half of that symbol at the time the cost reduction is applied (see rule 601.2e). If a colored half is chosen, the cost is reduced by one mana of that color. If a colorless half is chosen, the cost is reduced by an amount of generic mana equal to that half's number.
- 117.7d If a cost is reduced by an amount of mana represented by a Phyrexian mana symbol, the cost is reduced by one mana of that symbol's color.
- 117.8. Some spells and abilities have additional costs. An additional cost is a cost listed in a spell's rules text, or applied to a spell or ability from another effect, that its controller must pay at the same time that player pays the spell's mana cost or the ability's activation cost. A cost is an additional cost only if it's phrased using the word "additional." Note that some additional costs are listed in keywords; see rule 702.
- 117.8a Any number of additional costs may be applied to a spell as it's being cast or to an ability as it's being activated. The controller of the spell or ability announces his or her intentions to pay any or all of those costs as described in rule 601.2b.
- 117.8b Some additional costs are optional.
- 117.8c If an effect instructs a player to cast a spell "if able," and that spell has a mandatory additional cost that includes actions involving cards with a stated quality in a hidden zone, the player isn't required to cast that spell, even if those cards are present in that zone.
- 117.8d Additional costs don't change a spell's mana cost, only what its controller has to pay to cast it. Spells and abilities that ask for that spell's mana cost still see the original value.
- 117.8e Some effects increase the cost to cast a spell or activate an ability without using the word "additional." Those are not additional costs, and are not considered until determining the total cost of a spell or ability as described in rule 601.2e.
- 117.9. Some spells have alternative costs. An alternative cost is a cost listed in a spell's text, or applied to it from another effect, that its controller may pay rather than paying the spell's mana cost. Alternative costs are usually phrased, "You may [action] rather than pay [this object's] mana cost," or "You may cast [this object] without paying its mana cost." Note that some alternative costs are listed in keywords; see rule 702.
- 117.9a Only one alternative cost can be applied to any one spell as it's being cast. The controller of the spell announces his or her intentions to pay that cost as described in rule 601.2b.
- 117.9b Alternative costs are always optional.
- 117.9c An alternative cost doesn't change a spell's mana cost, only what its controller has to pay to cast it. Spells and abilities that ask for that spell's mana cost still see the original value.
- 117.9d If an alternative cost is being paid to cast a spell, any additional costs, cost increases, and cost reductions that affect that spell are applied to that alternative cost. (See rule 601.2e.)
- 117.10. Each payment of a cost applies to only one spell, ability, or effect. For example, a player can't sacrifice just one creature to activate the activated abilities of two permanents that each require sacrificing a creature as a cost. Also, the resolution of a spell or ability doesn't pay another spell or ability's cost, even if part of its effect is doing the same thing the other cost asks for.
- 117.11. The actions performed when paying a cost may be modified by effects. Even if they are, meaning the actions that are performed don't match the actions that are called for, the cost has still been paid.
Example: A player controls Psychic Vortex, an enchantment with a cumulative upkeep cost of "Draw a card," and Obstinate Familiar, a creature that says "If you would draw a card, you may skip that draw instead." The player may decide to pay Psychic Vortex’s cumulative upkeep cost and then draw no cards instead of drawing the appropriate amount. The cumulative upkeep cost has still been paid.
- 117.12. Some spells, activated abilities, and triggered abilities read, "[Do something]. If [a player] [does or doesn't], [effect]." or "[A player] may [do something]. If [that player] [does or doesn't], [effect]." The action [do something] is a cost, paid when the spell or ability resolves. The "If [a player] [does or doesn't]" clause checks whether the player chose to pay an optional cost or started to pay a mandatory cost, regardless of what events actually occurred.
Example: You control Standstill, an enchantment that says "When a player casts a spell, sacrifice Standstill. If you do, each of that player's opponents draws three cards." A spell is cast, causing Standstill's ability to trigger. Then an ability is activated that exiles Standstill. When Standstill's ability resolves, you're unable to pay the "sacrifice Standstill" cost. No player will draw cards.
Example: Your opponent has cast Gather Specimens, a spell that says "If a creature would enter the battlefield under an opponent's control this turn, it enters the battlefield under your control instead." You control a face-down Dermoplasm, a creature with morph that says "When Dermoplasm is turned face up, you may put a creature card with morph from your hand onto the battlefield face up. If you do, return Dermoplasm to its owner's hand." You turn Dermoplasm face up, and you choose to put a creature card with morph from your hand onto the battlefield. Due to Gather Specimens, it enters the battlefield under your opponent's control instead of yours. However, since you chose to pay the cost, Dermoplasm is still returned to its owner's hand.
- 117.12a Some spells, activated abilities, and triggered abilities read, "[Do something] unless [a player does something else]." This means the same thing as "[A player may do something else]. If [that player doesn't], [do something]."