Cheapest priced copy (Ikoria: Lair of Behemoths): $0.35 at Card Kingdom, near mint, their price list as of .
Startling Development 
Instant · Common · Jumpstart 2022 · #350
Until end of turn, target creature becomes a blue Serpent with base power and toughness 4/4.
Cycling {1} ({1}, Discard this card: Draw a card.)
Legal inCommanderPioneerModernLegacyVintagePauper
Not legal inBrawlStandard
2 printings · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Ikoria: Lair of Behemoths | 68 | Common | $0.35 | $0.49 |
| Jumpstart 2022 | 350 | Common | $0.35 | — |
4 rulings from Wizards of the Coast
- 17 April 2020Effects that modify the affected creature’s power or toughness without setting it will apply no matter when they started to take effect. The same is true for counters that change the creature’s power or toughness.
- 17 April 2020Startling Development overwrites all previous effects that set the affected creatures’ color, creature types, power, and/or toughness to specific values. Other effects that set these characteristics to specific values that start to apply after Startling Development resolves will overwrite that part of the effect.
- 17 April 2020The affected creature doesn’t lose any abilities when it becomes a Serpent.
- 17 April 2020Because damage remains marked on a creature until the damage is removed as the turn ends, nonlethal damage dealt to the target creature may become lethal once its base toughness becomes 4.