Mirran Mettle is a common Instant card from Mirrodin Besieged (MBS), collector number 84. Current market price: $0.19 (foil $0.55), TCGplayer market prices via Scryfall.
Target creature gets +2/+2 until end of turn. Metalcraft — That creature gets +4/+4 until end of turn instead if you control three or more artifacts.