Cheapest priced copy (Duel Decks: Elves vs. Inventors): $0.49 at Card Kingdom, near mint, their price list as of .
Sylvan Advocate 
Creature — Elf Druid Ally · 2/3 · Rare · Oath of the Gatewatch · #144
Vigilance
As long as you control six or more lands, this creature and land creatures you control get +2/+2.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
4 printings · $0.49 to $0.79 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Duel Decks: Elves vs. Inventors | 23 | Rare | $0.49 | — |
| Oath of the Gatewatch | 144 | Rare | $0.49 | $0.79 |
| Oath of the Gatewatch Promos | 144s | Rare | — | $0.49 |
| Zendikar Rising Commander | 84 | Rare | $0.49 | — |
2 rulings from Wizards of the Coast
- 22 January 2016A “land creature” is a permanent that’s both a land and a creature.
- 22 January 2016Damage remains marked on creatures until the turn ends. If Sylvan Advocate’s ability stops applying (because Sylvan Advocate leaves the battlefield or you no longer control six or more lands), then any land creatures that needed the toughness bonus to stay alive will be destroyed.