Cheapest priced copy (Eternal Masters): $0.35 at Card Kingdom, near mint, their price list as of .
Cephalid Sage 
Creature — Octopus · 2/3 · Common · Eternal Masters · #41
Threshold — As long as there are seven or more cards in your graveyard, this creature has "When this creature enters, draw three cards, then discard two cards."
Legal inCommanderLegacyVintagePauper
Not legal inBrawlStandardPioneerModern
2 printings · $0.35 to $0.79 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Eternal Masters | 41 | Common | $0.35 | $0.49 |
| Torment | 29 | Uncommon | $0.35 | $0.79 |
2 rulings from Wizards of the Coast
- 8 June 2016You draw three cards and discard two cards all while Cephalid Sage’s ability is resolving. Nothing can happen between the two, and no player may choose to take actions.
- 8 June 2016If you don’t have enough cards in your graveyard at the moment Cephalid Sage enters the battlefield, its threshold ability won’t trigger. If the number of cards in your graveyard changes and Cephalid Sage loses the ability once it has triggered, the ability will still resolve.