Cheapest priced copy (Eternal Masters): $0.35 at Card Kingdom, near mint, their price list as of .
Centaur Chieftain 
Creature — Centaur · 3/3 · Uncommon · Torment · #122
Haste
Threshold — As long as there are seven or more cards in your graveyard, this creature has "When this creature enters, creatures you control get +1/+1 and gain trample until end of turn."
Legal inCommanderLegacyVintage
Not legal inBrawlStandardPioneerModernPauper
2 printings · $0.35 to $1.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Eternal Masters | 160 | Uncommon | $0.35 | $0.49 |
| Torment | 122 | Uncommon | $0.35 | $1.99 |
2 rulings from Wizards of the Coast
- 8 June 2016All creatures under your control when Centaur Chieftain’s triggered ability resolves are affected, including Centaur Chieftain itself. Ones that come under your control or become creatures later in the turn are not.
- 8 June 2016If you don’t have enough cards in your graveyard at the moment Centaur Chieftain enters the battlefield, its threshold ability won’t trigger. If the number of cards in your graveyard changes and Centaur Chieftain loses the ability once it has triggered, the ability will still resolve.