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Whenever you cast a spell, this creature gets +X/+X until end of turn, where X is that spell's mana value.
Mana value takes into account only the mana symbols printed in the upper right corner of the card. Manaplasm doesn't care about additional costs, alternative costs, cost-reduction effects, or what you actually paid to cast the spell.
While a spell with {X} in its cost is on the stack, its mana value takes the chosen value of X into account. For example, if you cast Blaze (a spell with the mana cost {X}{R}) and choose a value of 5 for that X, then your Manaplasm will get +6/+6 until end of turn.
