Manaplasm

Creature — Ooze

Whenever you cast a spell, Manaplasm gets +X/+X until end of turn, where X is that spell's mana value.

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2008-10-01:
Converted mana cost 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.
2008-10-01:
While a spell with {X} 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 {X}{R}) and choose a value of 5 for that X, then your Manaplasm will get +6/+6 until end of turn.