Cheapest priced copy (Mirrodin Besieged): $0.35 at Card Kingdom, near mint, their price list as of .
Copper Carapace 
Artifact — Equipment · Common · Mirrodin Besieged · #102
Equipped creature gets +2/+2 and can't block.
Equip {3} ({3}: Attach to target creature you control. Equip only as a sorcery.)
Legal inCommanderModernLegacyVintagePauper
Not legal inBrawlStandardPioneer
3 printings · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Mirrodin Besieged | 102 | Common | $0.35 | $0.49 |
| Modern Masters 2015 | 205 | Common | $0.35 | $0.49 |
| The List | MM2-205 | Common | $0.35 | — |