Cheapest priced copy (Duel Decks: Jace vs. Vraska): $0.59 at Card Kingdom, near mint, their price list as of .
Jace's Phantasm 
Creature — Illusion · 1/1 · Common · Iconic Masters · #60
Flying
This creature gets +4/+4 as long as an opponent has ten or more cards in their graveyard.
Legal inCommanderModernLegacyVintagePauper
Not legal inBrawlStandardPioneer
4 printings · $0.59 to $2.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Duel Decks: Jace vs. Vraska | 3 | Uncommon | $0.59 | — |
| Iconic Masters | 60 | Common | $0.69 | $1.99 |
| Magic 2013 | 57 | Uncommon | $0.69 | $2.49 |
| The List | IMA-60 | Common | $0.69 | — |
3 rulings from Wizards of the Coast
- 17 November 2017If Jace's Phantasm is dealt lethal damage at the same time as an opponent's creature, and that creature will become that opponent's tenth card in their graveyard, Jace's Phantasm will be destroyed before it gets +4/+4. This is because all creatures that have been dealt lethal damage are destroyed at the same time.
- 17 November 2017In a multiplayer game, a single opponent must have ten or more cards in their graveyard for the bonus to apply, although this opponent may change over the course of the game. You don't have to pick a single opponent; the ability will monitor each opponent's graveyard.
- 17 November 2017The last ability of Jace's Phantasm constantly monitors each opponent's graveyard to see if the bonus applies. If the stated condition becomes no longer true, the bonus immediately stops applying.