Like most Pokmon cards, the value of Greninja BREAK 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 (BREAKpoint)
What started out as a neat new art treatment in Throne of Eldraine has become a tangled mess of trying to work out which cards can be found in which booster packs, and every set has so many different art styles vying for your attention that its easy to lose track
Its evolutionary line progresses into Prinplup and eventually into the majestic Empoleon, a Water/Steel type, which adds strategic depth in battles
Heartstone was in print in this block and then made the cut for this brew, but it helps your other stuff
Madison Li and then is used to power artefacts, buff robotic creatures like Rex, Cyber Hound and otherwise give you the upper hand through the power of applied science