6.Commanders begin the game in the Command Zone. While a commander is in the command zone, it may be cast, subject to the normal timing restrictions for casting creatures. Its owner must pay for each time it was previously cast from the command zone; this is an additional cost.
MTG Comprehensive Rules
601.2f The player determines the total cost of the spell. Usually this is just the mana cost. Some spells have additional or alternative costs. Some effects may increase or reduce the cost to pay, or may provide other alternative costs. Costs may include paying mana, tapping permanents, sacrificing permanents, discarding cards, and so on. The total cost is the mana cost or alternative cost (as determined in 601.2b), plus all additional costs and cost increases, and minus all cost reductions. If multiple cost reductions apply, the player may apply them in any order. If the mana component of the total cost is reduced to nothing by cost reduction effects, it is considered to be 0. It can’t be reduced to less than 0. Once the total cost is determined, any effects that directly affect the total cost are applied. Then the resulting total cost becomes "locked in." If effects would change the total cost after this time, they have no effect.
The total cost is the mana cost or alternative cost (as determined in 601.2b), plus all additional costs and cost increases, and minus all cost reductions.
Rule 601, "Casting Spells" —> Rule 601.2b:
If the spell is modal, the player announces the mode choice (see rule 700.2
). If the player wishes to splice any cards onto the spell (see rule 702.46
), he or she reveals those cards in his or her hand. If the spell has alternative or additional costs that will be paid as it’s being cast such as buyback or kicker costs (see rules 117.8
), the player announces his or her intentions to pay any or all of those costs (see rule 601.2f
). A player can’t apply two alternative methods of casting or two alternative costs to a single spell. If the spell has a variable cost that will be paid as it’s being cast (such as an
in its mana cost; see rule 107.3
), the player announces the value of that variable. If the value of that variable is defined in the text of the spell by a choice that player would make later in the announcement or resolution of the spell, that player makes that choice at this time instead of that later time. If a cost that will be paid as the spell is being cast includes hybrid mana symbols, the player announces the nonhybrid equivalent cost he or she intends to pay. If a cost that will be paid as the spell is being cast includes Phyrexian mana symbols, the player announces whether he or she intends to pay 2 life or the corresponding colored mana cost for each of those symbols. Previously made choices (such as choosing to cast a spell with flashback from a graveyard or choosing to cast a creature with morph face down) may restrict the player’s options when making these choices.
MTG Comprehensive Rules
702.139a Mutate appears on some creaturecards. It represents a static ability that functions while the spell with mutate is on the stack. “Mutate [cost]”means “You may pay [cost] rather than pay this spell’s mana cost. If you do, it becomes a mutating creature spelland targets a non-Human creature with the same owner as this spell.”Casting a spell using its mutate ability follows the rules for paying alternative costs