Cheapest priced copy (Double Masters): $0.35 at Card Kingdom, near mint, their price list as of .
Sylvan Might 
Instant · Uncommon · Odyssey · #277
Target creature gets +2/+2 and gains trample until end of turn.
Flashback {2}{G}{G} (You may cast this card from your graveyard for its flashback cost. Then exile it.)
Legal inCommanderLegacyVintagePauper
Not legal inBrawlStandardPioneerModern
4 printings · $0.35 to $1.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Double Masters | 181 | Common | $0.35 | $0.49 |
| Duel Decks: Mind vs. Might | 51 | Common | $0.35 | — |
| Eternal Masters | 188 | Common | $0.35 | $0.49 |
| Odyssey | 277 | Uncommon | $0.35 | $1.99 |
6 rulings from Wizards of the Coast
- 19 March 2021A spell cast using flashback will always be exiled afterward, whether it resolves, is countered, or leaves the stack in some other way.
- 19 March 2021“Flashback [cost]” means “You may cast this card from your graveyard by paying [cost] rather than paying its mana cost” and “If the flashback cost was paid, exile this card instead of putting it anywhere else any time it would leave the stack.”
- 19 March 2021To determine the total cost of a spell, start with the mana cost or alternative cost (such as a flashback cost) you're paying, add any cost increases, then apply any cost reductions. The mana value of the spell is determined only by its mana cost, no matter what the total cost to cast the spell was.
- 19 March 2021You must still follow any timing restrictions and permissions, including those based on the card's type. For instance, you can cast a sorcery using flashback only when you could normally cast a sorcery.
- 19 March 2021You can cast a spell using flashback even if it was somehow put into your graveyard without having been cast.
- 19 March 2021If a card with flashback is put into your graveyard during your turn, you can cast it if it's legal to do so before any other player can take any actions.