Cards that appear in winning lists hold value because players need copies
Like most Magic: The Gathering cards, the value of Zack Fair 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 (Final Fantasy)
This great Jeskai cards heyday in Modern has passed, but its still a potent source of card draw many Commander decks can leverage
Demonic Tutor Perhaps the best tutor card in all of Magic: The Gathering, Demonic Tutor is a sorcery and costs two mana, but it lets you fetch any card you need and use it right away
Its attempting to tone down the power level of poison as an alternate win condition while still being a viable path to build around
The main deciding factor for the price here is almost certainly art