Cost

A cost is a payment made for spells, activated abilities, or as part of effects. 
==Description==
Costs are very commonly paid by spending mana, but sometimes other resources are required such as life or energy counters, or actions are demanded such as sacrificing permanents or discarding cards. Activation costs for abilities frequently include tapping the permanent with the ability.
The total cost of a payment can be influenced by many effects and abilities, particularly when casting spells. It's primarily determined by the mana cost (or ability activation cost), but alternative costs, additional costs, and other cost increases or decreases can play a role. These modifications can come from abilities of the spell itself, or from external effects.
When an effect prompts a player to pay a cost, that cost could be required or optional, depending on the effect. Paying and/or not paying usually leads to additional effects.
===Payable costs===
A cost that requires specific actions implicitly grants players the ability to do things they wouldn't otherwise be able to do. For example, players normally can't sacrifice permanents for no reason, or put permanents into play without casting them. But if a cost demands those actions, then the player can simply follow the instruction of the cost and take those actions. Similarly, players can't normally reduce their life total at will, but if a cost demands a life payment (or any other resource), then the life can be paid.
Costs must be paid in full or not paid at all, and can only be paid if the necessary resources are available. This can be confusing for costs including actions that would be interpreted differently if they were simply effects. For example, "discard two cards" as a cost cannot be paid at all with only one card in hand, but "discard two cards" as an effect would cause the player to discard their one card.
Some effects explicitly prevent certain actions from being taken, like "cards can't leave graveyards". These would prevent relevant costs from being paid, like "exile this card from your graveyard".
On the other hand, replacement effects which change the results of cost-paying actions don't prevent the cost from being paid. For example, "if a creature would die, exile it instead" still allows you to sacrifice creatures, they just go to the exile zone instead of the graveyard. "If any number of counters would be put on a permanent, half that many counters are put on it instead" still allows you to "pay the cost" of putting a -1/-1 counter on a creature, even though no counters are actually placed.
===Total cost===
Total cost	=	Mana cost orActivation cost 	+	Additional costs	+	Misc. cost increases		Misc. cost decreases
Total cost	=	OR 	+	Additional costs	+	Misc. cost increases		Misc. cost decreases
Total cost	=	Alternate cost(includes "without paying mana cost")	+	Additional costs	+	Misc. cost increases		Misc. cost decreases
The total cost for a payment is a starting cost combined with any cost increases and decreases that may apply. The total cost includes all actions and resources required, not just mana.
# Spells almost always have a mana cost, while activated abilities have an activation cost. Some effects or spells allow for an alternative cost to be paid instead of the normal mana cost. This includes casting it "without paying its mana cost", which means the cost starts at 0 before additions and reductions. If one or more alternative costs are available, only one can be chosen and paid, and any abilities that depend on an alternative cost being paid will only work with the corresponding alternative cost.
# All remaining cost modifiers are applied after the starting cost, first increases, then decreases. Additional costs are a form of increase that usually comes from the spell being cast. Other miscellaneous cost increases or decreases can be caused by various effects and are often phrased as "[thing] costs more [or less] to cast".
Notably, casting "without paying its mana cost" may still result in a total cost which includes mana payments, if it's due to a cost increase.
The total cost can be quite different from the spell's printed mana cost, and may include amounts or types of mana different from that mana cost. Despite this, the characteristic of that spell called "mana cost" is still the same. Only the "cost paid to cast it" is different.
Only after the total cost is determined is the payment made, all at once and in full. If the total cost can't be paid, then no sub-part of the cost can be paid either, even if it's a mandatory cost. For example:
* Any crew ability has an activation cost of tapping creatures with a total power above some threshold. Its controller cannot "try" to activate it by tapping creatures with lesser power. If the card is under the influence of an <c>Anointed Peacekeeper</c> which increases the activation cost by , its controller cannot tap the creatures even if they do have sufficient power, unless they pay the extra to pay the full cost.
* A mandatory cost like "...exile two cards from your graveyard. If you can't..." will not force the player to exile one card from the graveyard if they only have one - it simply proceeds to the consequences because they couldn't exile two.See rulings for <c>Egon, God of Death</c>.
===Mandatory costs===
Some effects require a player to pay a cost if they are able to, usually with consequences if they cannot pay it. In this case, the player cannot willingly choose to take the consequences by not paying. (On the other hand, if the text says they "may" pay the cost, it is not mandatory.) Almost always, mandatory costs consist of actions involving permanents or cards, such as sacrificing permanents or exiling cards from the graveyard, and not payment of mana. In these cases it is clear whether the player can take the action or not - e.g. if they control the permanents or have the cards in graveyard, they can sacrifice or exile them, and must do so if mandatory.
A mandatory cost never forces players to take actions other than the cost itself, even if those actions would then allow the cost to be paid. In particular, mana abilities are never required to be used in order to pay a mandatory cost. Therefore, a player may feel like they could pay a mandatory cost like because they have an untapped land, but the game considers them to be "unable" to pay it at that time because they do not already have the required mana. Therefore, they are not required to pay it. (They may still add the mana and thus become able to pay it if they like.)
This situation is rare since mana payments aren't designed into mandatory costs, but could occur due to cost increases. An instruction to cast a spell "if able" means that spell's cost is mandatory. Most effects like this provide an alternative cost (including "without paying its mana cost") so that mana payment is not an issue, but cost increases to the spell that do include mana are the most common cause of this situation.
==Rules==
==References==