Cheapest priced copy (The Brothers' War Retro Artifacts): $0.35 at Card Kingdom, near mint, their price list as of .
Sigil of Valor 
Artifact — Equipment · Uncommon · Magic Origins · #239
Whenever equipped creature attacks alone, it gets +1/+1 until end of turn for each other creature you control.
Equip {1} ({1}: Attach to target creature you control. Equip only as a sorcery.)
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
5 printings · $0.35 to $0.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| The Brothers' War Retro Artifacts | 53 | Uncommon | $0.35 | $0.49 |
| The Brothers' War Retro Artifacts | 116 | Uncommon | $0.35 | $0.49 |
| The Brothers' War Retro Artifacts | 116z | Uncommon | — | — |
| Magic Origins | 239 | Uncommon | $0.35 | $0.99 |
| The List | ORI-239 | Uncommon | $0.35 | — |
2 rulings from Wizards of the Coast
- 22 June 2015Count the number of creatures you control other than the equipped creature as Sigil of Valor's ability resolves to determine the amount of the bonus. Once the ability resolves, the bonus won't change, even if the number of creatures you control does.
- 22 June 2015A creature attacks alone if it's the only creature declared as an attacker during the declare attackers step (including creatures controlled by your teammates, if applicable). For example, Sigil of Valor's ability won't trigger if you attack with multiple creatures and all but one of them are removed from combat.