Cheapest priced copy (Foundations Jumpstart): $0.35 at Card Kingdom, near mint, their price list as of .
Relic Axe 
Artifact — Equipment · Uncommon · Zendikar Rising · #248
When this Equipment enters, attach it to target creature you control.
Equipped creature gets +1/+1. If it's a Warrior, it gets +2/+1 instead.
Equip {2} ({2}: Attach to target creature you control. Equip only as a sorcery.)
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
2 printings · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Foundations Jumpstart | 759 | Uncommon | $0.35 | — |
| Zendikar Rising | 248 | Uncommon | $0.35 | $0.49 |
2 rulings from Wizards of the Coast
- 25 September 2020If the target creature becomes an illegal target, the Equipment remains on the battlefield unattached.
- 25 September 2020Attaching an Equipment with its enters-the-battlefield triggered ability isn't the same as using its equip ability. You don't pay mana for the attachment, and if the Equipment enters at a time you couldn't cast a sorcery, you can still attach it to a creature you control.