Cheapest priced copy (Adventures in the Forgotten Realms): $0.39 at Card Kingdom, near mint, their price list as of .
Dragon's Fire 
Instant · Common · Adventures in the Forgotten Realms · #139
As an additional cost to cast this spell, you may reveal a Dragon card from your hand or choose a Dragon you control.
Dragon's Fire deals 3 damage to target creature or planeswalker. If you revealed a Dragon card or chose a Dragon as you cast this spell, Dragon's Fire deals damage equal to the power of that card or creature instead.
Legal inCommanderPioneerModernLegacyVintagePauper
Not legal inBrawlStandard
2 printings · $0.39 to $0.79 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Adventures in the Forgotten Realms | 139 | Common | $0.39 | $0.79 |
| Foundations Jumpstart | 134 | Common | $0.59Out of stock | — |
2 rulings from Wizards of the Coast
- 23 July 2021A Dragon card is a card with the creature type Dragon in its type line. Similarly, a creature on the battlefield is a Dragon if it has the Dragon creature type. A card that has "Dragon" in the name (such as Dragon's Fire) isn't a Dragon card unless it also has the Dragon creature type.
- 23 July 2021If you chose a Dragon you control, use that Dragon's power as Dragon's Fire resolves to determine how much damage is dealt. If the Dragon you chose is no longer on the battlefield as Dragon's Fire resolves, Dragon's Fire will deal damage equal to the power it had the last time it was on the battlefield.