Cheapest priced copy (Theros Beyond Death Promos): $0.49 at Card Kingdom, near mint, their price list as of .
Ashiok's Erasure 
Enchantment · Rare · Theros Beyond Death · #304
Flash
When this enchantment enters, exile target spell.
Your opponents can't cast spells with the same name as the exiled card.
When this enchantment leaves the battlefield, return the exiled card to its owner's hand.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
4 printings · $0.49 to $0.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Theros Beyond Death Promos | 43p | Rare | $0.49 | $0.79 |
| Theros Beyond Death Promos | 43s | Rare | — | $0.59 |
| Theros Beyond Death | 43 | Rare | $0.49 | $0.49 |
| Theros Beyond Death | 304 | Rare | $0.49 | $0.99 |
5 rulings from Wizards of the Coast
- 24 January 2020If Ashiok's Erasure has exiled an adventurer card, your opponents can't cast spells with the same name as the creature, but may cast their Adventures.
- 24 January 2020The exiled spell isn't countered, but it won't resolve. This works against spells that can't be countered.
- 24 January 2020If Ashiok's Erasure has exiled a split card, your opponents can't cast a spell with the same name as either half of that card.
- 24 January 2020If Ashiok's Erasure leaves the battlefield before its enters-the-battlefield ability has resolved, its last ability triggers and resolves with no effect, then its enters-the-battlefield ability exiles the target spell for the rest of the game.
- 24 January 2020If there is no exiled card (perhaps because the exiled spell was a copy of a spell), or if the exiled card leaves exile somehow, Ashiok's Erasure won't stop players from casting spells.