Cheapest priced copy (Throne of Eldraine): $3.49 at Card Kingdom, near mint, their price list as of .
The Magic Mirror 
Legendary Artifact · Mythic · Throne of Eldraine Promos · #51p
This spell costs {1} less to cast for each instant and sorcery card in your graveyard.
You have no maximum hand size.
At the beginning of your upkeep, put a knowledge counter on The Magic Mirror, then draw a card for each knowledge counter on The Magic Mirror.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
4 printings · $3.49 to $11.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Throne of Eldraine | 51 | Mythic | $3.49 | $5.99 |
| Throne of Eldraine | 345 | Mythic | $7.49 | $11.99 |
| Throne of Eldraine Promos | 51p | Mythic | $5.99 | $6.99 |
| Throne of Eldraine Promos | 51s | Mythic | — | $10.99 |
6 rulings from Wizards of the Coast
- 4 October 2019If a split card is both an instant card and a sorcery card, it's only counted once for The Magic Mirror's cost reduction ability.
- 4 October 2019Once you announce that you're casting a spell, no player may take actions until the spell has been paid for. Notably, opponents can't try to change by how much a relic's cost is reduced.
- 4 October 2019The cost reduction ability reduces only the generic mana in the relic's cost. The colored mana must still be paid.
- 4 October 2019If The Magic Mirror leaves the battlefield while its last ability is on the stack, the ability will use the number of knowledge counters The Magic Mirror had before leaving the battlefield to determine how many cards you'll draw.
- 4 October 2019Adventurer cards aren't instant or sorcery cards while they're in your graveyard.
- 4 October 2019To determine the total cost of a spell, start with the mana cost or alternative cost you're paying, add any cost increases, then apply any cost reductions. The mana value of the spell remains unchanged, no matter what the total cost to cast it was.