Cheapest priced copy (March of the Machine): $0.35 at Card Kingdom, near mint, their price list as of .
Protocol Knight 
Creature — Human Knight · 3/4 · Common · March of the Machine · #74
When this creature enters, tap target creature an opponent controls. Put a stun counter on that creature if you control another Knight. (If a permanent with a stun counter would become untapped, remove one from it instead.)
Legal inCommanderPioneerModernLegacyVintagePauper
Not legal inBrawlStandard
1 printing · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| March of the Machine | 74 | Common | $0.35 | $0.49 |
2 rulings from Wizards of the Coast
- 14 April 2023Whether or not you control another Knight is checked only as Protocol Knight’s ability is resolving. It doesn’t matter if you still control Protocol Knight at that time, or if it’s still a Knight.
- 14 April 2023Protocol Knight’s ability can target a creature that’s already tapped.