Cheapest priced copy (Duel Decks: Elves vs. Inventors): $0.49 at Card Kingdom, near mint, their price list as of .
Gladehart Cavalry 
Creature — Elf Knight · 6/6 · Rare · Oath of the Gatewatch · #132
When this creature enters, support 6. (Put a +1/+1 counter on each of up to six other target creatures.)
Whenever a creature you control with a +1/+1 counter on it dies, you gain 2 life.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
4 printings · $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Duel Decks: Elves vs. Inventors | 11 | Rare | $0.49 | — |
| Oath of the Gatewatch | 132 | Rare | $0.49 | $0.49 |
| Oath of the Gatewatch Promos | 132 | Rare | — | $0.49 |
| Oath of the Gatewatch Promos | 132s | Rare | — | $0.49 |
6 rulings from Wizards of the Coast
- 22 January 2016You can’t put more than one +1/+1 counter on any one target using the support action.
- 22 January 2016If a spell with support has other abilities that target creatures, those abilities and the support ability can target the same creature.
- 22 January 2016If Gladehart Cavalry dies at the same time as a creature you control with a +1/+1 counter on it, its ability will trigger.
- 22 January 2016If Gladehart Cavalry somehow gets a +1/+1 counter, it dying will cause its own ability to trigger.
- 22 January 2016You gain 2 life per creature, not per +1/+1 counter.
- 22 January 2016Support can target a creature you don’t control.