Cheapest priced copy (Dominaria United): $2.29 at Card Kingdom, near mint, their price list as of .
Karn, Living Legacy 
Legendary Planeswalker — Karn · Mythic · Dominaria United · #372
+1: Create a tapped Powerstone token. (It's an artifact with "{T}: Add {C}. This mana can't be spent to cast a nonartifact spell.")
−1: Pay any amount of mana. Look at that many cards from the top of your library, then put one of those cards into your hand and the rest on the bottom of your library in a random order.
−7: You get an emblem with "Tap an untapped artifact you control: This emblem deals 1 damage to any target."
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
4 printings · $2.29 to $9.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Dominaria United | 1 | Mythic | $2.29 | $2.49 |
| Dominaria United | 372 | Mythic | $2.49 | $3.99 |
| Dominaria United Promos | 1p | Mythic | $6.49 | $9.99 |
| Dominaria United Promos | 1s | Mythic | — | $3.99 |
7 rulings from Wizards of the Coast
- 9 September 2022For Karn’s second loyalty ability, you don’t choose how much mana to pay until the ability resolves. Once the ability begins to resolve and you have paid, no player may respond until you have finished all the steps of that ability.
- 9 September 2022Karn’s emblem has an activated ability that allows you to tap untapped artifacts to pay its cost. Notably, you may tap artifact creatures that entered the battlefield this turn to pay this cost, even if they don’t have haste.
- 9 September 2022Powerstone tokens are a new kind of predefined token. Each one has the artifact subtype “Powerstone” and the ability “{T}: Add {C}. This mana can’t be spent to cast a nonartifact spell.”
- 9 September 2022If you don’t pay any mana, you won’t look at any cards or put any into your hand.
- 9 September 2022You can use the {C} added by a Powerstone token on anything that isn’t a nonartifact spell. This includes paying costs to activate abilities of both artifact and nonartifact permanents, paying ward costs, and so on.
- 9 September 2022If you look at one or more cards this way, you must put a card into your hand. You can’t choose not to put one into your hand.
- 9 September 2022Although all the cards in the Dominaria United set that create Powerstone tokens create a tapped Powerstone token, entering the battlefield tapped isn’t part of the token’s definition. Notably, if you create a token that is a copy of a Powerstone token, the token copy won’t enter the battlefield tapped.