Cheapest priced copy (Conspiracy: Take the Crown): $0.35 at Card Kingdom, near mint, their price list as of .
Manaplasm 
Creature — Ooze · 1/1 · Rare · Shards of Alara · #138
Whenever you cast a spell, this creature gets +X/+X until end of turn, where X is that spell's mana value.
Legal inCommanderModernLegacyVintage
Not legal inBrawlStandardPioneerPauper
2 printings · $0.35 to $1.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Shards of Alara | 138 | Rare | $0.49 | $1.49 |
| Conspiracy: Take the Crown | 186 | Uncommon | $0.35 | $1.49 |
2 rulings from Wizards of the Coast
- 1 October 2008Mana 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.
- 1 October 2008While 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.