Cheapest priced copy (Fate Reforged): $0.35 at Card Kingdom, near mint, their price list as of .
Dragonrage 
Instant · Uncommon · Fate Reforged · #97
Add {R} for each attacking creature you control. Until end of turn, attacking creatures you control gain "{R}: This creature gets +1/+0 until end of turn."
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
1 printing · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Fate Reforged | 97 | Uncommon | $0.35 | $0.49 |
3 rulings from Wizards of the Coast
- 24 November 2014Only creatures that are attacking as Dragonrage resolves will count toward how much mana is generated, and only those creatures will gain the "firebreathing" activated ability. In other words, casting Dragonrage before you've declared attackers usually won't do anything.
- 24 November 2014Even though it can generate mana, Dragonrage isn't a mana ability. It uses the stack and can be responded to.
- 24 November 2014Remember that unused mana is lost at the end of each step and phase. For example, if you cast Dragonrage during your declare attackers step, except in very rare cases, the mana Dragonrage generates won't last until the declare blockers step or the postcombat main phase.