While a spell with in its cost is on the stack, its converted mana cost takes the chosen value of X into account. For example, if you cast Blaze (a spell with the mana cost ) and choose a value of 5 for that X, then your Manaplasm will get +6/+6 until end of turn.
Converted mana cost takes into account only the mana symbols printed in the upper right corner of the card. Manaplasm doesnâ€™t care about additional costs, alternative costs, cost-reduction effects, or what you actually paid to cast the spell.
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