Cheapest priced copy (Mirrodin Besieged): $0.35 at Card Kingdom, near mint, their price list as of .
Spire Serpent 
Creature — Serpent · 3/5 · Common · Mirrodin Besieged · #32
Defender
Metalcraft — As long as you control three or more artifacts, this creature gets +2/+2 and can attack as though it didn't have defender.
Legal inCommanderModernLegacyVintagePauper
Not legal inBrawlStandardPioneer
1 printing · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Mirrodin Besieged | 32 | Common | $0.35 | $0.49 |
2 rulings from Wizards of the Coast
- 1 June 2011Spire Serpent still has defender if you control three or more artifacts, although it will be able to attack.
- 1 June 2011If Spire Serpent has been declared as an attacker, and you stop controlling three or more artifacts later in combat, Spire Serpent continues being an attacking creature until combat ends.