Cheapest priced copy (Oath of the Gatewatch): $0.35 at Card Kingdom, near mint, their price list as of .
Seed Guardian 
Creature — Elemental · 3/4 · Uncommon · Oath of the Gatewatch · #143
Reach
When this creature dies, create an X/X green Elemental creature token, where X is the number of creature cards in your graveyard.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
1 printing · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Oath of the Gatewatch | 143 | Uncommon | $0.35 | $0.49 |
2 rulings from Wizards of the Coast
- 22 January 2016Use the number of creature cards in your graveyard as the ability resolves to determine the value of X. Seed Guardian will count toward this number as long as it’s still in your graveyard at that time.
- 22 January 2016Once the Elemental is created, the number of creature cards in your graveyard changing won’t cause the Elemental’s power or toughness to change.