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 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 ) and choose a value of 5 for that X, then your Manaplasm will get +6/+6 until end of turn.