Cheapest priced copy (Ravnica Allegiance): $0.35 at Card Kingdom, near mint, their price list as of .
Sphinx's Insight 
Instant · Common · Ravnica Allegiance · #209
Draw two cards.
Addendum — If you cast this spell during your main phase, you gain 2 life.
Legal inCommanderPioneerModernLegacyVintagePauper
Not legal inBrawlStandard
1 printing · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Ravnica Allegiance | 209 | Common | $0.35 | $0.49 |
2 rulings from Wizards of the Coast
- 12 January 2024Addendum abilities of instant spells apply while the spell is resolving, not immediately after casting it. If the spell is countered, you don't get the addendum bonus.
- 12 January 2024If an effect copies a spell with an addendum ability while it's on the stack, the copy wasn't cast at all, so you won't get the addendum bonus.