Cheapest priced copy (Duel Decks: Jace vs. Vraska): $0.59 at Card Kingdom, near mint, their price list as of .
Jace's Mindseeker 
Creature — Fish Illusion · 4/4 · Rare · Duel Decks: Jace vs. Vraska · #19
Flying
When this creature enters, target opponent mills five cards. You may cast an instant or sorcery spell from among them without paying its mana cost.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
2 printings · $0.59 to $0.79 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Duel Decks: Jace vs. Vraska | 19 | Rare | $0.59 | — |
| Magic 2014 | 61 | Rare | $0.79 | $0.79 |
6 rulings from Wizards of the Coast
- 1 July 2013When casting an instant or sorcery card this way, ignore timing restrictions based on the card’s type. Other timing restrictions, such as “Cast [this spell] only during combat,” must be followed.
- 1 July 2013If the five cards are put into a public zone such as exile instead of a graveyard (perhaps due to the ability of Rest in Peace), you can cast one of those instant or sorcery cards from that zone.
- 1 July 2013If you cast an instant or sorcery card this way, you do so while the ability of Jace’s Mindseeker is resolving. If you choose not to (or can’t), you won’t get a chance to cast one later.
- 1 July 2013If you cast a card “without paying its mana cost,” you can’t pay alternative costs such as overload costs. You can pay additional costs such as kicker costs. If the card has mandatory additional costs, you must pay those.
- 1 July 2013“From among them” means the five cards put into the graveyard, not all cards in that graveyard.
- 1 July 2013If a card has {X} in its mana cost, you must choose 0 as its value.