Cheapest priced copy (Journey into Nyx): $0.35 at Card Kingdom, near mint, their price list as of .
Mortal Obstinacy 
Enchantment — Aura · Common · Journey into Nyx · #17
Enchant creature you control
Enchanted creature gets +1/+1.
Whenever enchanted creature deals combat damage to a player, you may sacrifice this Aura. If you do, destroy target enchantment.
Legal inCommanderPioneerModernLegacyVintagePauper
Not legal inBrawlStandard
1 printing · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Journey into Nyx | 17 | Common | $0.35 | $0.49 |
2 rulings from Wizards of the Coast
- 26 April 2014If another player gains control of the enchanted creature, Mortal Obstinacy will be put into its owner's graveyard.
- 26 April 2014You decide whether to sacrifice Mortal Obstinacy as its triggered ability resolves. If it's not on the battlefield at that time, you can't sacrifice it and destroy the target enchantment.