Cheapest priced copy (Ikoria: Lair of Behemoths Promos): $0.49 at Card Kingdom, near mint, their price list as of .
Extinction Event 
Sorcery · Rare · Ikoria: Lair of Behemoths · #321
Choose odd or even. Exile each creature with mana value of the chosen quality. (Zero is even.)
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
6 printings · $0.49 to $8.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Ikoria: Lair of Behemoths | 88 | Rare | $0.59 | $0.79 |
| Ikoria: Lair of Behemoths | 321 | Rare | $0.69 | $2.99 |
| Marvel Universe | 65 | Mythic | $2.79 | $8.99 |
| Ikoria: Lair of Behemoths Promos | 88p | Rare | $0.49 | $1.49 |
| Ikoria: Lair of Behemoths Promos | 88s | Rare | — | $1.49 |
| The List | IKO-88 | Rare | — | — |
3 rulings from Wizards of the Coast
- 17 April 2020The mana value of a token that isn't a copy of another object is 0. A token that is a copy of another object has the same mana cost as that object.
- 17 April 2020If a creature on the battlefield has {X} in its mana cost, X is considered to be 0.
- 17 April 2020You choose odd or even while Extinction Event is resolving. Players can't take any actions between the time you make that choice and the time some creatures go extinct.