The most important factors are edition (1st Edition, Shadowless, Unlimited), condition (mint cards command premiums), rarity (holographic, secret rare, full art), age (vintage Base Set cards are highly sought after), and the Pokemon itself (Charizard, Pikachu, and fan favorites are worth more)
By the way, you wouldn't happen to have $3,000 we can borrow to play Reanimator in Legacy, would you?
In the mainline games, Gyarados has been a staple due to its intimidating presence and high attack stats, making it a favored choice for trainers who wish to dominate the competitive scene
Here is how the supply chain actually works
Finding one is the collector's equivalent of a perfect game
Remove three study counters from this enchantment, Sacrifice this enchantment: Draw seven cards