Like most Pokmon cards, the value of Mewtwo-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 (BW Black Star Promos)
Winning enough matches would earn a team this holographic promo card, which reflected the parent-child format with an image of kangaroo-like Kangaskhan with an infant Kangaskhan in its stomach pouch
Every card that you put into your hand was another card worth having, even if you had no use for it
Aside from Pikachu, there's also Rowlet, Growlithe, Glossifleur, Smeargle, Bewear, Chinchou, Cutiefly, Drampa, Flaafy, Victini, Tynamo, Pangoro, Lapras and Ledyba
Traditional Foil Mythic Rare in 1% of boosters
"Pokmon Trading Card Game Online Review"