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It's not just gameplay utility that makes these cards costly -- it's the difficulty of finding physical copies of those cards, hence players' constant demands for reprints in Masters sets and even Commander deck cycles

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Blog posts Japanese Pokemon Booster MEGA Expansion Pokemon TCG Pre-Order Storm Emeralda Japanese Storm Emeralda Pre-Order: The Complete Buyer's Guide to the New MEGA Pokemon Booster By PokeNinJapan A complete buyer's guide to the Japanese Storm Emeralda (M6) MEGA expansion pack, covering release details, key cards, and how to secure a pre-order before it sells out

That explains the demand for this card, and it's also likely to be in fairly limited supply, given that it was only printed in the Warhammer Commander decks, which always get snapped up swiftly when a new batch is released