Ferrovore [Scars of Mirrodin] noPrice where X is the amount

where X is the amount of mana that player paid this way

" —Ashling

Set: Starter 1999 Type: Sorcery Rarity: Rare Cost: {5}{R}{R} Fire Tempest deals 6 damage to each creature and each player

Set: Conspiracy Type: Artifact Creature — Construct Rarity: Rare Cost: {6} Draft Cogwork Grinder face up

but the reverse may be closer to the truth

Ferrovore [Scars of Mirrodin] noPrice where X is the amountSet: Scars of Mirrodin Type: Creature Beast Rarity: Common Cost: {2}{R} {R}, Sacrifice an artifact: Ferrovore gets +3 +0 until end of turn. The Vulshok use its digestion to break down the most obstinate artifacts, from darksteel myr to seastrider plates.