Cheapest priced copy (Magic 2014): $0.49 at Card Kingdom, near mint, their price list as of .
Haunted Plate Mail 
Artifact — Equipment · Rare · Magic 2015 · #218
Equipped creature gets +4/+4.
{0}: Until end of turn, this permanent becomes a 4/4 Spirit artifact creature that's no longer an Equipment. Activate only if you control no creatures.
Equip {4} ({4}: Attach to target creature you control. Equip only as a sorcery.)
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
3 printings · $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Magic 2014 | 212 | Rare | $0.49 | $0.49 |
| Magic 2015 | 218 | Rare | $0.49 | $0.49 |
| The List | M14-212 | Rare | $0.49 | — |
3 rulings from Wizards of the Coast
- 18 July 2014You can activate the equip ability of Haunted Plate Mail while it isn't an Equipment, but the ability will have no effect. Haunted Plate Mail won't become attached to any creature.
- 18 July 2014Although you must control no creatures to activate the "animation" ability, having a creature somehow come under your control in response to that ability won't stop it from resolving.
- 18 July 2014It's possible to activate the first activated ability of Haunted Plate Mail in response to activating the first ability of another Haunted Plate Mail and have them both become creatures until end of turn.