Like most Pokmon cards, the value of Greninja 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 (Twilight Masquerade)
Investment Thesis HeartGold SoulSilver is a vintage set with strong historical performance and low reprint risk, making it a reliable long-term investment
Thats an intensely powerful ability, especially combined with everything else on this list
To top it all off, this card features Metagross' shiny form, which stands out as a major bonus to this card's cool factor, as if it wasn't high enough already
More card advantage and quality effects offsets this quite well
Ive experienced this firsthand with my Aurelia deck