I just threw in every card I thought could be good and could never feasibly end up cutting 80+ cards when I considered them all to be 'good.' The trick that I now use to efficiently and effectively build decks in that format is to build portions of my deck (mostly the Trainers) while ignoring singleton rules adding four Professor's Research, four Marnie, a couple of Cynthia, four Quick Balls, a couple of Pokmon Communication, etc. as placeholders
Like most Pokmon cards, the value of Mega Dragonite 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)
From Aristocrats to Sacrifice there are tons of archetypes this is a must include in
If you're not looking for one specific Pokmon card, there are countless other options
However, there aren't anywhere near as many 1st Edition Base Set packs out there as regular Base Set packs, hence the major price difference