Celebi is best preserved as a collection piece, after all
Like most Magic: The Gathering cards, the value of Redwood Treefolk depends on a few consistent factors: its rarity, the printing and finish (regular or foil), its physical condition, and the set it was printed in (Portal)
For instance, in Hearthstone there are Minion, Spell, Weapon, and Secret cards, each one having its own perks
If youre really playing the game, youre just trying to get all the cards, most of which are not expensive. The prices of Pokmon cards in gem mint grade are starting to stabilize, according to Horvath
Graded Population Report: Clair's TM 01 #117 Visualize the distrbution of grades in the chart
The cards seem to have been stolen during the production phase when they're stored in boxes (the secret rares are printed separately from other cards because of their texture)