Cheapest priced copy (Guilds of Ravnica): $7.49 at Card Kingdom, near mint, their price list as of .
Mnemonic Betrayal 
Sorcery · Mythic · Guilds of Ravnica · #189
Exile all opponents' graveyards. You may cast spells from among those cards this turn, and mana of any type can be spent to cast them. At the beginning of the next end step, if any of those cards remain exiled, return them to their owners' graveyards.
Exile Mnemonic Betrayal.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
1 printing · $7.49 to $139.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Guilds of Ravnica | 189 | Mythic | $7.49 | $139.99 |
5 rulings from Wizards of the Coast
- 5 October 2018Mnemonic Betrayal doesn’t change when you can cast the exiled cards. For example, if you exile a sorcery card, you can cast it only during your main phase when the stack is empty.
- 5 October 2018You can’t play land cards exiled this way.
- 5 October 2018Only the cards that remain in exile are returned to their owners’ graveyards. For example, a permanent spell cast this way will remain on the battlefield.
- 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 Mnemonic Betrayal’s effect are exiled. If you leave the game before Mnemonic Betrayal’s delayed triggered ability resolves, the cards remain exiled.
- 5 October 2018If you cast a spell this way and that card is exiled, it’s considered a new object. Mnemonic Betrayal no longer allows you to cast it. It remains in exile when Mnemonic Betrayal’s delayed triggered ability resolves.