Cheapest priced copy (Duel Decks: Elspeth vs. Kiora): $0.35 at Card Kingdom, near mint, their price list as of .
Time to Feed 
Sorcery · Common · Duel Decks: Elspeth vs. Kiora · #50
Choose target creature an opponent controls. When that creature dies this turn, you gain 3 life. Target creature you control fights that creature. (Each deals damage equal to its power to the other.)
Legal inCommanderPioneerModernLegacyVintagePauper
Not legal inBrawlStandard
5 printings · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Duel Decks: Elspeth vs. Kiora | 50 | Common | $0.35 | — |
| Foundations Jumpstart | 727 | Common | $0.35 | — |
| Jumpstart | 438 | Common | $0.35 | — |
| The List | DDO-50 | Common | $0.35 | — |
| Theros | 181 | Common | $0.35 | $0.49 |
3 rulings from Wizards of the Coast
- 15 September 2013If neither creature is a legal target when Time to Feed tries to resolve, the spell won't resolve and none of its effects will happen.
- 15 September 2013If the first target creature dies that turn, you'll gain 3 life no matter what caused the creature to die or who controls the creature at that time.
- 15 September 2013Time to Feed has two targets: a creature an opponent controls and a creature you control. If only one of those creatures is a legal target when Time to Feed tries to resolve, the creatures won't fight and neither will deal or be dealt damage. If the creature you don't control is the illegal target, you won't gain life when it dies.