Cheapest priced copy (Commander Legends: Battle for Baldur's Gate): $0.35 at Card Kingdom, near mint, their price list as of .
Cloudkill 
Sorcery · Uncommon · Commander Legends: Battle for Baldur's Gate · #121
All creatures get -X/-X until end of turn, where X is the greatest mana value of a commander you own on the battlefield or in the command zone.
Legal inCommanderLegacyVintage
Not legal inBrawlStandardPioneerModernPauper
2 printings · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Commander Legends: Battle for Baldur's Gate | 121 | Uncommon | $0.35 | $0.49 |
| The List | CLB-121 | Uncommon | — | — |
One ruling from Wizards of the Coast
- 10 June 2022If your commander is under the control of another player as Cloudkill resolves, it is still on the battlefield and will still be used to determine the value of X.