Cheapest priced copy (Modern Masters 2015): $0.35 at Card Kingdom, near mint, their price list as of .
Rusted Relic 
Artifact · Common · Modern Masters 2015 · #227
Metalcraft — This artifact is a 5/5 Golem artifact creature as long as you control three or more artifacts.
Legal inCommanderModernLegacyVintagePauper
Not legal inBrawlStandardPioneer
2 printings · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Modern Masters 2015 | 227 | Common | $0.35 | $0.49 |
| Scars of Mirrodin | 199 | Uncommon | $0.35 | $0.49 |
2 rulings from Wizards of the Coast
- 1 January 2011Whether Rusted Relic has "summoning sickness" is based on how long you have continuously controlled it, not on how long it has continuously been a creature.
- 1 January 2011If Rusted Relic stops being a creature during combat (because you lose control of one or more artifacts, perhaps), it is removed from combat and will not deal or be dealt combat damage. Creatures it was blocking remain blocked. It won't re-enter that combat, even if you wind up controlling three or more artifacts again during that combat phase.