Cheapest priced copy (Adventures in the Forgotten Realms): $0.35 at Card Kingdom, near mint, their price list as of .
Mordenkainen's Polymorph 
Instant · Common · Adventures in the Forgotten Realms · #65
Until end of turn, target creature becomes a Dragon with base power and toughness 4/4 and gains flying. (It loses all other creature types.)
Legal inCommanderPioneerModernLegacyVintagePauper
Not legal inBrawlStandard
1 printing · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Adventures in the Forgotten Realms | 65 | Common | $0.35 | $0.49 |
2 rulings from Wizards of the Coast
- 23 July 2021Mordenkainen's Polymorph will overwrite any previous effects that set the creature's power and toughness to specific values. Effects that otherwise modify the target creature's power and toughness will still apply no matter when they took effect. The same is true for +1/+1 counters.
- 23 July 2021The creature doesn't lose any of its abilities.