Cheapest priced copy (Kamigawa: Neon Dynasty): $0.35 at Card Kingdom, near mint, their price list as of .
Ancestral Katana 
Artifact — Equipment · Common · Kamigawa: Neon Dynasty · #1
Whenever a Samurai or Warrior you control attacks alone, you may pay {1}. When you do, attach this Equipment to it.
Equipped creature gets +2/+1.
Equip {3} ({3}: 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 |
|---|---|---|---|---|
| Kamigawa: Neon Dynasty | 1 | Common | $0.35 | $0.49 |
2 rulings from Wizards of the Coast
- 18 February 2022If you choose to pay {1} as Ancestral Katana's ability resolves, a second triggered ability triggers and players may respond to it. Then, as that second ability resolves, Ancestral Katana becomes attached to the creature that attacked alone.
- 18 February 2022A creature attacks alone if it's the only creature declared as an attacker during the declare attackers step. For example, Ancestral Katana's triggered ability won't trigger if you attack with multiple creatures and all but one of them are removed from combat.