Cheapest priced copy (Commander 2019): $0.49 at Card Kingdom, near mint, their price list as of .
Biomass Mutation 
Instant · Rare · Commander 2019 · #187
Creatures you control have base power and toughness X/X until end of turn.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
10 printings · $0.49 to $1.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Commander 2019 | 187 | Rare | $0.49 | — |
| Commander 2021 | 209 | Rare | $0.49 | — |
| Duskmourn: House of Horror Commander | 214 | Rare | $0.49 | — |
| Gatecrash | 213 | Rare | $0.59 | $0.79 |
| Fallout | 212 | Rare | $0.49 | $0.49 |
| Fallout | 475 | Rare | $0.49 | $0.49 |
| Fallout | 740 | Rare | — | $1.49 |
| Fallout | 1003 | Rare | — | $0.99 |
| The List | GTC-213 | Rare | $0.49 | — |
| Secrets of Strixhaven Commander | 297 | Rare | $0.49 | — |
2 rulings from Wizards of the Coast
- 24 January 2013Biomass Mutation overwrites any effects that set the power and/or toughness of a creature you control to a specific value. Effects that modify power and/or toughness but don't set them to a specific value (like the one created by Giant Growth), power/toughness changes from counters, and effects that switch a creature's power and toughness will continue to apply. For example, say you control a 1/1 creature that's getting +4/+2 from an Aura, has a +1/+1 counter on it, and is affected by an effect that switches its power and toughness. This creature is 4/6. If you cast Biomass Mutation with X equal to 4, the creature would become 7/9 (4/4, +4/+2 from the Aura, +1/+1 from the counter, then switch power and toughness).
- 24 January 2013Choosing 0 as the value for X will likely cause creatures you control to become 0/0 and be put into the graveyard.