Cheapest priced copy (Conspiracy: Take the Crown): $0.35 at Card Kingdom, near mint, their price list as of .
Keepsake Gorgon 
Creature — Gorgon · 2/5 · Uncommon · Theros · #93
Deathtouch
{5}{B}{B}: Monstrosity 1. (If this creature isn't monstrous, put a +1/+1 counter on it and it becomes monstrous.)
When this creature becomes monstrous, destroy target non-Gorgon creature an opponent controls.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
2 printings · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Conspiracy: Take the Crown | 141 | Uncommon | $0.35 | $0.49 |
| Theros | 93 | Uncommon | $0.35 | $0.49 |
3 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 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.