Cheapest priced copy (Khans of Tarkir): $0.35 at Card Kingdom, near mint, their price list as of .
Kin-Tree Invocation 
Sorcery · Uncommon · The List · #KTK-183
Create an X/X black and green Spirit Warrior creature token, where X is the greatest toughness among creatures you control.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
2 printings · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Khans of Tarkir | 183 | Uncommon | $0.35 | $0.49 |
| The List | KTK-183 | Uncommon | $0.35 | — |
2 rulings from Wizards of the Coast
- 20 September 2014If you control no creatures as Kin-Tree Invocation resolves, you’ll put a 0/0 token onto the battlefield. Unless something else is raising its toughness above 0, it will immediately be put into its owner’s graveyard and then cease to exist. Any abilities that trigger when a creature enters the battlefield or dies will trigger.
- 20 September 2014The value of X is determined only once, as Kin-Tree Invocation resolves. The power and toughness of the Spirit Warrior token doesn’t later change if the greatest toughness among creatures you control changes.