Cheapest priced copy (Fallout): $5.99 at Card Kingdom, near mint, their price list as of .
Mutational Advantage 
Instant · Rare · Secret Lair Drop · #2454
Permanents you control with counters on them gain hexproof and indestructible until end of turn. Prevent all damage that would be dealt to those permanents this turn. Proliferate. (Choose any number of permanents and/or players, then give each another counter of each kind already there.)
Legal inCommanderLegacyVintage
Not legal inBrawlStandardPioneerModernPauper
5 printings · $5.99 to $12.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Fallout | 111 | Rare | $5.99 | $7.99 |
| Fallout | 422 | Rare | $7.99 | $7.99 |
| Fallout | 639 | Rare | — | $9.99 |
| Fallout | 950 | Rare | — | $12.99 |
| Secret Lair Drop | 2454 | Rare | $7.99 | $8.99 |
6 rulings from Wizards of the Coast
- 8 March 2024If a player or permanent has more than one kind of counter on it, and you choose for it to get additional counters, it must get one of each kind of counter it already has. You can’t have it get just one kind of counter it already has and not the others.
- 8 March 2024Players can respond to a spell or ability whose effect includes proliferating. Once that spell or ability starts to resolve, however, and its controller chooses which permanents and players will get new counters, it’s too late for anyone to respond.
- 8 March 2024You can choose any permanent that has a counter, including ones controlled by opponents. You can choose any player who has a counter, including opponents. You can’t choose cards in any zone other than the battlefield, even if they have counters on them.
- 8 March 2024Some spells and abilities that cause you to proliferate may require targets. If each target chosen is an illegal target as that spell or ability tries to resolve, it won’t resolve. You won’t proliferate.
- 8 March 2024You don’t have to choose every permanent or player that has a counter—only the ones you want to add counters to. Since “any number” includes zero, you don’t have to choose any permanents at all, and you don’t have to choose any players at all.
- 8 March 2024The set of permanents affected by Mutational Advantage is determined at the time Mutational Advantage resolves. Permanents that gain counters later in the turn won’t become affected by this effect, and permanents that lose all of their counters later in the turn won’t stop being affected.