Cheapest priced copy (Strixhaven: School of Mages): $0.35 at Card Kingdom, near mint, their price list as of .
Mage Duel 
Sorcery · Common · Strixhaven: School of Mages · #137
This spell costs {2} less to cast if you've cast another instant or sorcery spell this turn.
Target creature you control gets +1/+2 until end of turn. Then it fights target creature you don't control. (Each deals damage equal to its power to the other.)
Legal inCommanderPioneerModernLegacyVintagePauper
Not legal inBrawlStandard
1 printing · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Strixhaven: School of Mages | 137 | Common | $0.35 | $0.49 |
3 rulings from Wizards of the Coast
- 16 April 2021To determine the total cost of a spell, start with the mana cost or alternative cost you’re paying, add any cost increases, then apply any cost reductions (such as that of Mage Duel). Mage Duel’s mana value is always 3, no matter what the total cost to cast it was.
- 16 April 2021If the creature you control is an illegal target as Mage Duel tries to resolve, that creature won’t get +1/+2. If that creature is a legal target, but the one you don’t control isn’t, your creature will still get +1/+2. In both cases, no fight will happen and no creature will deal or be dealt damage.
- 16 April 2021You can’t cast Mage Duel unless you choose both a creature you control and a creature you don’t control as targets.