Manaplasm
Creature — Ooze
Whenever you cast a spell, Manaplasm gets +X/+X until end of turn, where X is that spell's converted mana cost.
Urak froze when he heard it. That was his first mistake. He turned and cast a dramatic ward spell. That was his last.
Illustrated by
Conspiracy: Take the Crown#186UncommonEnglishLegality
Rulings
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.
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.