Cheapest priced copy (Tarkir: Dragonstorm Commander): $0.99 at Card Kingdom, near mint, their price list as of .
Stormbreath Dragon 
Creature — Dragon · 4/4 · Mythic · Theros · #143
Flying, haste, protection from white
{5}{R}{R}: Monstrosity 3. (If this creature isn't monstrous, put three +1/+1 counters on it and it becomes monstrous.)
When this creature becomes monstrous, it deals damage to each opponent equal to the number of cards in that player's hand.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
2 printings · $0.99 to $5.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Tarkir: Dragonstorm Commander | 237 | Mythic | $0.99 | — |
| Theros | 143 | Mythic | $0.99 | $5.99 |
4 rulings from Wizards of the Coast
- 15 September 2013Once a creature becomes monstrous, it can't become monstrous again. If the creature is already monstrous when the monstrosity ability resolves, nothing happens.
- 15 September 2013Monstrous isn't an ability that a creature has. It's just something true about that creature. If the creature stops being a creature or loses its abilities, it will continue to be monstrous.
- 15 September 2013For each opponent, count the number of cards in that player's hand when the last ability resolves to determine how much damage Stormbreath Dragon does to them.
- 15 September 2013An ability that triggers when a creature becomes monstrous won't trigger if that creature isn't on the battlefield when its monstrosity ability resolves.