Cheapest priced copy (Commander Legends): $16.99 at Card Kingdom, near mint, their price list as of .
Mnemonic Deluge 
Sorcery · Mythic · Commander Legends · #638
Exile target instant or sorcery card from a graveyard. Copy that card three times. You may cast the copies without paying their mana costs. Exile Mnemonic Deluge.
Legal inCommanderLegacyVintage
Not legal inBrawlStandardPioneerModernPauper
2 printings · $16.99 to $27.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Commander Legends | 82 | Mythic | $16.99 | $17.99Out of stock |
| Commander Legends | 638 | Mythic | $21.99 | $27.99Out of stock |
4 rulings from Wizards of the Coast
- 10 November 2020You create and cast the copies all during the resolution of Mnemonic Deluge. You can't wait and cast them later. If you don't cast all three copies, the ones you don't cast will cease to exist.
- 10 November 2020Because you're casting the spells during the resolution of Mnemonic Deluge, ignore any timing restrictions based on card type. In other words, you can cast copies of a sorcery card this way.
- 10 November 2020If you cast a spell "without paying its mana cost," you can't choose to cast it for any alternative costs. You can, however, pay additional costs. If the card has any mandatory additional costs, you must pay those to cast it.
- 10 November 2020If the card you copy has {X} in its mana cost, you must choose 0 as the value of X when casting the copy without paying its mana cost.