Like most Magic: The Gathering cards, the value of Dee Kay, Finder of the Lost 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 (Unfinity)
But the real treasure is its ETB ability that lets you swipe an instant or sorcery from an opponents graveyard, exile it, and cast it that turn using any mana, making it beneficial for attrition or grindy matches
They're usually best used at end of turn on the turn before you, that way you can respond to anything that happens, and if you happen to have mana left over before your turn, draw some cards
You'll have to wait a bit longer before buying them yourself, though, as they won't go on sale until November 5, and then end on November 8.
To be clear, Im one of the folks who doesnt enjoy the incredibly unique experience of playing with silver-bordered cards
So Im forced into the realm of guesswork and estimation: Based on what we hear from sellers and distributors, plus the likely print run after Fallout, were looking at around 1.5 million packs, which means youve got a 0.13% chance of pulling any serialized card