Cheapest priced copy (Innistrad: Crimson Vow): $0.49 at Card Kingdom, near mint, their price list as of .
Alchemist's Gambit 
Sorcery · Rare · Innistrad: Crimson Vow · #374
Cleave {4}{U}{U}{R} (You may cast this spell for its cleave cost. If you do, remove the words in square brackets.)
Take an extra turn after this one. During that turn, damage can't be prevented. [At the beginning of that turn's end step, you lose the game.]
Exile Alchemist's Gambit.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
4 printings · $0.49 to $6.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Innistrad: Double Feature | 407 | Rare | $0.79 | $6.99 |
| Innistrad: Crimson Vow Promos | 140s | Rare | — | $1.99 |
| Innistrad: Crimson Vow | 140 | Rare | $0.49 | $0.79 |
| Innistrad: Crimson Vow | 374 | Rare | $0.49 | $1.99 |
7 rulings from Wizards of the Coast
- 19 November 2021If you cast a spell for its cleave cost, that spell doesn't have any of the text in square brackets while it's on the stack.
- 19 November 2021Exiling Alchemist's Gambit happens as part of its resolution. If the spell is countered, it will go to the graveyard as normal.
- 19 November 2021If you somehow skip the extra turn Alchemist's Gambit gives you or skip that turn's end step, the delayed triggered ability never triggers. (If you cast Alchemist's Gambit by paying its cleave cost, the delayed triggered ability is never created at all.)
- 19 November 2021If multiple “extra turn” effects resolve in the same turn, the most recently created turn will be taken first.
- 19 November 2021You can't cast a spell for both its cleave cost and another alternative cost. For example, if an effect gives an Alchemist's Retrieval in your graveyard a flashback cost of {U}, you can't cast it from your graveyard for its cleave cost.
- 19 November 2021If an effect allows you to “cast a spell without paying its mana cost,” you can't cast that spell for its cleave cost.
- 19 November 2021A cleave cost is an alternative cost that's paid instead of the spell's mana cost. Casting a spell for its cleave cost doesn't change the spell's mana value.