Cheapest priced copy (War of the Spark): $0.35 at Card Kingdom, near mint, their price list as of .
Rubblebelt Rioters 
Creature — Human Berserker · 0/4 · Uncommon · War of the Spark · #215
Haste
Whenever this creature attacks, it gets +X/+0 until end of turn, where X is the greatest power among creatures you control.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
1 printing · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| War of the Spark | 215 | Uncommon | $0.35 | $0.49 |
2 rulings from Wizards of the Coast
- 3 May 2019If Rubblebelt Rioters is the only creature you control as its triggered ability resolves, X is equal to its own power. Unless another effect or a +1/+1 counter has raised its power above 0, this won’t have any effect.
- 3 May 2019The value of X is determined only as the triggered ability of Rubblebelt Rioters resolves. Once that happens, the value of X won’t change later in the turn even if the greatest power among creatures you control changes.