Cheapest priced copy (Throne of Eldraine): $0.35 at Card Kingdom, near mint, their price list as of .
Slaying Fire 
Instant · Uncommon · Throne of Eldraine · #143
Slaying Fire deals 3 damage to any target.
Adamant — If at least three red mana was spent to cast this spell, it deals 4 damage instead.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
2 printings · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Throne of Eldraine | 143 | Uncommon | $0.35 | $0.49 |
| Throne of Eldraine | 394 | Uncommon | $0.35 | $0.49 |
4 rulings from Wizards of the Coast
- 4 October 2019If an effect allows you to cast a spell without paying its mana cost, you can't choose to cast it and pay unless another rule or effect allows you to cast that spell for a cost. Similarly, you can't waive a cost reduction unless that effect says you may.
- 4 October 2019If an instant or sorcery spell has an adamant ability, you perform the spell's instructions in order. You don't perform the adamant instruction before the spell resolves or before any other effects printed above it.
- 4 October 2019If you copy a spell that has an adamant ability, no mana was spent to cast the copy at all, so that ability won't apply.
- 4 October 2019Adamant effects check what mana was actually spent to cast a spell. If an effect allows you to spend mana “as though it were mana” of any color or type, that allows you to spend mana you couldn't otherwise spend, but it doesn't change what mana you spent to cast the spell.