Cheapest priced copy (Strixhaven: School of Mages): $0.99 at Card Kingdom, near mint, their price list as of .
Exponential Growth 
Sorcery · Rare · Strixhaven: School of Mages · #130
Until end of turn, double target creature's power X times.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
4 printings · $0.99 to $3.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Strixhaven: School of Mages Promos | 130p | Rare | $3.49 | $3.99 |
| Strixhaven: School of Mages Promos | 130s | Rare | — | $2.79 |
| Strixhaven: School of Mages | 130 | Rare | $0.99 | $0.99 |
| Strixhaven: School of Mages | 318 | Rare | $1.49 | $3.49 |
3 rulings from Wizards of the Coast
- 16 April 2021Exponential Growth doubles a creature’s power sequentially, not all at the same time. For example, if X is 3 and you target a 2/2 creature, you will double its power once to 4, again to 8, and a third time to 16, making it a 16/2 creature until end of turn.
- 16 April 2021If a creature’s power is less than 0 when it’s doubled, instead that creature gets -X/-0, where X is how much less than 0 its power is. For example, if an effect has given a 5/5 creature -7/-0 so that it’s a -2/5 creature, doubling its power makes it a -4/5 creature.
- 16 April 2021If an effect instructs you to “double” a creature’s power, that creature gets +X/+0, where X is its power as that effect begins to apply. (This is not the value of X you chose when you cast the spell)