ETCHEDCheapest priced copy (March of the Machine: The Aftermath): $0.49 at Card Kingdom, near mint, their price list as of .
Arni Metalbrow 
Legendary Creature — Human Berserker · 3/3 · Rare · March of the Machine: The Aftermath · #116
Whenever a creature you control attacks or enters attacking, you may pay {1}{R}. If you do, you may put a creature card with mana value less than that creature's mana value from your hand onto the battlefield tapped and attacking.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
5 printings · $0.49 to $3.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| March of the Machine: The Aftermath | 16 | Rare | $0.49 | $0.49 |
| March of the Machine: The Aftermath | 66 | Rare | $0.69 | $0.49 |
| March of the Machine: The Aftermath | 116 | Rare | — | — |
| March of the Machine: The Aftermath | 159 | Rare | $0.59 | $0.49 |
| March of the Machine: The Aftermath | 200 | Rare | — | $3.49 |
3 rulings from Wizards of the Coast
- 12 May 2023Putting a creature onto the battlefield attacking as the ability resolves will cause the ability to trigger again. This instance of the ability will consider the mana value of the new creature that entered the battlefield attacking.
- 12 May 2023You decide whether to pay {1}{R} as the triggered ability resolves.
- 12 May 2023If the attacking creature or creature that entered the battlefield attacking isn't on the battlefield as the triggered ability resolves, use its mana value from when it was last on the battlefield. (If it's on the battlefield but is no longer attacking, use its current mana value.)