Like most Pokmon cards, the value of Gyarados-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 (XY Black Star Promos)
Machamp ex has slightly higher HP and a slightly higher attack making it that bit more powerful than the standard version of it, but this also means you arent really missing out on much by not having the ex Pokemon
While Mill X cards like Wall of Lost Thoughts will churn through four cards for two mana, which isnt a terrible mana to mill ratio, without top deck knowledge theres no way of knowing what exactly youre putting in your graveyard
Even the hallowed Jace was bad at release since there wasnt a single other playable blue card in the format and Bloodbraid Elf + Blightning threatened to shut him down as soon as he resolved
Occasionally you'll want ten and seven
Find one Retro or Schematic artifact card with Retro Magic frames in each booster