Cheapest priced copy (March of the Machine: The Aftermath): $21.99 at Card Kingdom, near mint, their price list as of .
Karn, Legacy Reforged 
Legendary Artifact Creature — Golem · */* · Mythic · March of the Machine: The Aftermath · #99
Karn's power and toughness are each equal to the greatest mana value among artifacts you control.
At the beginning of your upkeep, add {C} for each artifact you control. This mana can't be spent to cast nonartifact spells. Until end of turn, you don't lose this mana as steps and phases end.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
5 printings · $19.99 to $44.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| March of the Machine: The Aftermath | 49 | Mythic | $21.99 | $19.99 |
| March of the Machine: The Aftermath | 99 | Mythic | $29.99Out of stock | $22.99 |
| March of the Machine: The Aftermath | 149 | Mythic | — | — |
| March of the Machine: The Aftermath | 184 | Mythic | $22.99 | $32.99 |
| Mystery Booster 2 | 255 | Mythic | — | $44.99 |
3 rulings from Wizards of the Coast
- 12 May 2023The mana produced by Karn's last ability can be spent on anything that isn't a nonartifact spell. This includes casting artifact spells, paying costs to activate abilities of both artifact and nonartifact permanents, paying ward costs, and so on.
- 12 May 2023Karn's last ability isn't a mana ability, even though it adds mana. It uses the stack and it can be responded to. Use the number of artifacts you control as the ability resolves to determine how much mana to add.
- 12 May 2023The ability that defines Karn's power and toughness works in all zones, not just the battlefield. As long as Karn is under your control and still an artifact, its own mana value will count. In most cases, it'll be at least 5/5.