Cheapest priced copy (Outlaws of Thunder Junction Commander): $3.49 at Card Kingdom, near mint, their price list as of .
Baleful Mastery 
Instant · Rare · Strixhaven: School of Mages · #64
You may pay {1}{B} rather than pay this spell's mana cost.
If the {1}{B} cost was paid, an opponent draws a card.
Exile target creature or planeswalker.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
6 printings · $3.49 to $9.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Outlaws of Thunder Junction Commander | 126 | Rare | $3.49 | — |
| The List | STX-64 | Rare | $3.99 | — |
| Strixhaven: School of Mages Promos | 64p | Rare | $3.49 | $4.49 |
| Strixhaven: School of Mages Promos | 64s | Rare | — | $7.49 |
| Strixhaven: School of Mages | 64 | Rare | $3.49 | $4.99 |
| Strixhaven: School of Mages | 301 | Rare | $4.49Out of stock | $9.99 |
5 rulings from Wizards of the Coast
- 16 April 2021 If you copy a "Mastery" spell and the alternative cost was paid, the copy will resolve as though the cost was paid.
- 16 April 2021 The mana value of a spell on the stack is determined by its mana cost, not any alternative costs you used to pay for it.
- 16 April 2021 In a multiplayer game, you choose which opponent takes the prescribed action as the spell resolves.
- 16 April 2021 If you choose to pay one alternative cost, you can't pay any other alternative costs. For example, if an effect lets you cast a "Mastery" spell "without paying its mana cost," you can't also choose to pay its given alternative cost.
- 16 April 2021 If an effect increases or decreases the cost of spells you cast, that cost increase or decrease is applied to the alternative cost you chose to pay. In that case, the cost was still paid for the purposes of the effect, even if you paid more or less for it when it was cast.