Cheapest priced copy (Modern Masters): $0.35 at Card Kingdom, near mint, their price list as of .
Erratic Mutation 
Instant · Common · Planar Chaos · #41
Choose target creature. Reveal cards from the top of your library until you reveal a nonland card. That creature gets +X/-X until end of turn, where X is that card's mana value. Put all cards revealed this way on the bottom of your library in any order.
Legal inCommanderModernLegacyVintagePauper
Not legal inBrawlStandardPioneer
3 printings · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Modern Masters | 42 | Common | $0.35 | $0.49 |
| Planar Chaos | 41 | Common | $0.35 | $0.49 |
| Time Spiral Remastered | 67 | Common | $0.35 | $0.49 |
2 rulings from Wizards of the Coast
- 19 March 2021If a card in a player's library has {X} in its mana cost, X is considered to be 0.
- 19 March 2021You reveal cards and the creature gets +X/-X all while Erratic Mutation is resolving. Nothing can happen in between, and no player can take actions.