Cheapest priced copy (Adventures in the Forgotten Realms): $0.35 at Card Kingdom, near mint, their price list as of .
Spare Dagger 
Artifact — Equipment · Common · Adventures in the Forgotten Realms · #250
Equipped creature gets +1/+0 and has "Whenever this creature attacks, you may sacrifice Spare Dagger. When you do, this creature deals 1 damage to any target."
Equip {1} ({1}: Attach to target creature you control. Equip only as a sorcery.)
Legal inCommanderPioneerModernLegacyVintagePauper
Not legal inBrawlStandard
1 printing · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Adventures in the Forgotten Realms | 250 | Common | $0.35 | $0.49 |