Cheapest priced copy (Amonkhet): $0.35 at Card Kingdom, near mint, their price list as of .
Cartouche of Strength 
Enchantment — Aura Cartouche · Common · The List · #AKH-158
Enchant creature you control
When this Aura enters, you may have enchanted creature fight target creature an opponent controls. (Each deals damage equal to its power to the other.)
Enchanted creature gets +1/+1 and has trample.
Legal inCommanderPioneerModernLegacyVintagePauper
Not legal inBrawlStandard
2 printings · $0.35 to $0.99 at Card Kingdom
6 rulings from Wizards of the Coast
- 18 April 2017If Cartouche of Strength leaves the battlefield before its triggered ability resolves, the creature it last enchanted will fight the target creature if it's still on the battlefield at that time. However, it won't have +1/+1 from Cartouche of Strength anymore.
- 18 April 2017Each Cartouche is an Aura with "Enchant creature you control." If another player gains control of either the Cartouche or the enchanted creature (but not both), then the Cartouche will be enchanting an illegal permanent and be put into its owner's graveyard as a state-based action.
- 18 April 2017The enchanted creature has +1/+1 and trample while it's fighting. However, trample doesn't apply during a fight.
- 18 April 2017If the triggered ability's target is illegal when it tries to resolve, or if the enchanted creature has left the battlefield, no creature will deal or be dealt damage.
- 18 April 2017Cartouche is an enchantment subtype with no special meaning. Other cards may care about which enchantments are Cartouches.
- 18 April 2017If the target creature becomes an illegal target before a Cartouche spell resolves, the spell doesn't resolve. It doesn't enter the battlefield.