Cheapest priced copy (Neon Dynasty Commander): $2.99 at Card Kingdom, near mint, their price list as of .
Mage Slayer 
Artifact — Equipment · Uncommon · Alara Reborn · #57
Whenever equipped creature attacks, it deals damage equal to its power to the player or planeswalker it's attacking.
Equip {3}
Legal inCommanderModernLegacyVintage
Not legal inBrawlStandardPioneerPauper
4 printings · $2.99 to $17.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Alara Reborn | 57 | Uncommon | $3.49 | $17.99 |
| Planechase | 91 | Uncommon | $3.99Out of stock | — |
| Neon Dynasty Commander | 140 | Uncommon | $2.99 | — |
| The List | HOP-91 | Uncommon | — | — |
2 rulings from Wizards of the Coast
- 1 May 2009This ability triggers and resolves in the declare attackers step. The creature will still deal its combat damage as normal later on during the combat phase.
- 1 May 2009When the equipped creature attacks, the first ability will trigger. When that ability resolves, that creature will deal damage equal to its power to player or planeswalker it's attacking, even if Mage Slayer has left the battlefield or is equipping a different creature by that time.