Cheapest priced copy (Dominaria United): $1.79 at Card Kingdom, near mint, their price list as of .
Karn's Sylex 
Legendary Artifact · Mythic · Dominaria United Promos · #234p
Karn's Sylex enters tapped.
Players can't pay life to cast spells or to activate abilities that aren't mana abilities.
{X}, {T}, Exile Karn's Sylex: Destroy each nonland permanent with mana value X or less. Activate only as a sorcery.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
3 printings · $1.49 to $6.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Dominaria United | 234 | Mythic | $1.79 | $1.49 |
| Dominaria United Promos | 234p | Mythic | $6.49 | $4.99 |
| Dominaria United Promos | 234s | Mythic | — | $3.99 |
5 rulings from Wizards of the Coast
- 9 September 2022If a spell or an activated ability that isn’t a mana ability has a cost that requires a player to pay life, that spell or ability can’t be cast or activated.
- 9 September 2022Other things may still cause players to pay life, such as a resolving spell or ability.
- 9 September 2022Players may always pay 0 life, even if an effect says they can’t pay life.
- 9 September 2022Activated abilities contain a colon. They’re generally written “[Cost]: [Effect].” Some keyword abilities are activated abilities (such as cycling) and will have colons in their reminder text.
- 9 September 2022An activated mana ability is one that produces mana as it resolves, not one that costs mana to activate.