Like most Magic: The Gathering cards, the value of Fangorn, Tree Shepherd 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 (The Lord of the Rings: Tales of Middle-earth)
There is a token sub-theme since you create so many Clue tokens that allow you to gain extra advantage from them
Its a suitably powerful effect for the most iconic Wizard of all time, and it opens up a mind-boggling array of deckbuilding possibilities, especially when you factor in his ability to grant your other legendary permanent flash
The rest are likely to stick around the $1 mark for quite a while, though
You could also go a tokens route, generating tokens for casting non-creature spells, and using effects like Impact Tremors to kill opponents and keep your hand topped up