Like most Pokmon cards, the value of Registeel 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 (Legend Maker)
Like plenty of other Eldrazi, its difficult to reprint these in Standard or accessible sets, so the price tends to drive up as there is always demand for the big Eldrazi
Once it's used, it cannot be used again for the rest of the game unless the creature is removed from the battlefield and returned to it
This makes everything pop and gives your cards a professional finish
Something I forgot to mention back in 2017s article was that an ally-colored cycle of precons had gone unreleased by the time they switched to the biodomes
On TCGPlayer, listings for an Enchanted Donald Duck - Pie Slinger start at $220, with many hovering around $250