Like most Pokmon cards, the value of Mega Gengar 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 (Ascended Heroes)
PSE Prismatic Secret Rare Soul Energy MAX
We would recommend unlocking each deck just to get a feel for them and add them all to your collection, but if you want to jump straight into battles with real opponents, focus on the Black & White: Basic Blue, Red and Green decks as they give you a bunch of handy trainer cards for building your own deck
Card Q&A Q2684 Mar
Duelists can re-submit Deck Lists submitted online, but only to accommodate for a new Forbidden & Limited List or a new product release
Foil cards will weigh more than their non-foil variants and, when weighing, make sure you are comparing like for like