Back in the old days of Magic (like ten years ago), single card prices were based mostly on their age, availability, and playability
Creature - Soldier Spirit Color tag templates(None) Dust Animus Flying If you control five or more untapped lands, this creature enters with two +1/+1 counters and a lifelink counter on it
In the context of The Brothers War, the new keyword is excellent at providing you a way to make use of Powerstones while not punishing you for leaving all your rocks in the car before battle, which means a wide variety of Commander decks will likely be seeking to include Prototype cards in their lists
Meanwhile, a standard English booster pack is guaranteed to have at least 3 foil cards, but the rarity and value of those foil cards is far more random
A little less than a week later, though, the card is right back at the top of the leaderboard, this time, alongside a very surprising partner