Cheapest priced copy (Outlaws of Thunder Junction Commander): $0.99 at Card Kingdom, near mint, their price list as of .
Thief of Sanity 
Creature — Specter · 2/2 · Rare · Guilds of Ravnica · #205
Flying
Whenever this creature deals combat damage to a player, look at the top three cards of that player's library, exile one of them face down, then put the rest into their graveyard. You may cast that card for as long as it remains exiled, and mana of any type can be spent to cast that spell.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
7 printings · $0.99 to $4.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Ravnica: Clue Edition | 212 | Rare | $1.29 | — |
| Guilds of Ravnica | 205 | Rare | $1.29 | $3.99 |
| New Capenna Commander | 354 | Rare | $1.29 | — |
| Outlaws of Thunder Junction Commander | 243 | Rare | $0.99 | — |
| Guilds of Ravnica Promos | 205p | Rare | $2.79 | $4.99 |
| Guilds of Ravnica Promos | 205s | Rare | — | $4.49Out of stock |
| The List | OTC-243 | Rare | — | — |
5 rulings from Wizards of the Coast
- 5 October 2018Casting an exiled card causes it to leave exile. You can't cast it multiple times.
- 5 October 2018You can still cast the exiled card if Thief of Sanity leaves the battlefield or leaves your control. If another player gains control of Thief of Sanity, that player can't cast the exiled card.
- 5 October 2018An effect that instructs you to "cast" a card doesn't allow you to play lands.
- 5 October 2018Thief of Sanity's effect doesn't change when you can cast the exiled card. For example, if you exile a sorcery card, you can cast it only during your main phase when the stack is empty.
- 5 October 2018In a multiplayer game, if a player leaves the game, all cards that player owns leave as well. If you leave the game, any spells or permanents you control from Thief of Sanity's effect are exiled, and any of the face-down exiled cards remain face down indefinitely. No player may look at them.