Not all rare cards are expensive
Like most Pokmon cards, the value of Shaymin-EX 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 (Legendary Treasures)
Banlist History Restriction changes recorded for Duality
It was Standard-defining to the point of damaging the format as a whole, and was the single biggest card on most players' radars to remove from the format
These packs are based around a theme, which is typically 1 or more archetypes, and pull from a featured card pool of 40 or fewer cards to help build a Deck based around that theme, in addition to several cards from the Master Pack
But, like many cards that this process produces, the mechanics are a bit scattered and without synergy