Cheapest priced copy (Amonkhet): $0.35 at Card Kingdom, near mint, their price list as of .
River Serpent 
Creature — Serpent · 5/5 · Common · The List · #AKH-66
This creature can't attack unless there are five or more cards in your graveyard.
Cycling {U} ({U}, Discard this card: Draw a card.)
Legal inCommanderPioneerModernLegacyVintagePauper
Not legal inBrawlStandard
2 printings · $0.35 to $0.49 at Card Kingdom
One ruling from Wizards of the Coast
- 18 April 2017Once River Serpent has attacked, it will remain in combat even if the number of cards in your graveyard becomes four or fewer.