Cheapest priced copy (Conspiracy: Take the Crown): $0.35 at Card Kingdom, near mint, their price list as of .
Nessian Asp 
Creature — Snake · 4/5 · Uncommon · Conspiracy: Take the Crown · #187
Reach
{6}{G}: Monstrosity 4. (If this creature isn't monstrous, put four +1/+1 counters on it and it becomes monstrous.)
Legal inCommanderPioneerModernLegacyVintagePauper
Not legal inBrawlStandard
3 printings · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Conspiracy: Take the Crown | 187 | Uncommon | $0.35 | $0.49 |
| Duel Decks: Elspeth vs. Kiora | 48 | Common | $0.35 | — |
| Theros | 164 | Common | $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 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.
- 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.