FOILCheapest priced copy (Shadows over Innistrad): $0.99 at Card Kingdom, near mint, their price list as of .
Wolf of Devil's Breach 
Creature — Elemental Wolf · 5/5 · Mythic · Shadows over Innistrad Promos · #192s
Whenever this creature attacks, you may pay {1}{R} and discard a card. If you do, this creature deals damage to target creature or planeswalker equal to the discarded card's mana value.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
2 printings · $0.99 to $2.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Shadows over Innistrad Promos | 192s | Mythic | — | $2.49 |
| Shadows over Innistrad | 192 | Mythic | $0.99 | $1.99 |
3 rulings from Wizards of the Coast
- 8 April 2016You choose the target creature or planeswalker as the triggered ability of Wolf of Devil’s Breach is put onto the stack. You choose which card to discard, if any, as the ability resolves. While players may respond to the triggered ability once you’ve chosen a target, no player may take actions between the time you discard the card and the time damage is dealt.
- 8 April 2016You can’t choose to discard a card without also paying {1}{R}.
- 8 April 2016If a card with {X} in its mana cost is discarded to Wolf of Devil’s Breach’s ability, X is considered to be 0.