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)
I feel price are inflated and the comparison long term with an Espeon ex (which is a personal favourite) its hard to validate
There are so many directions for you to take your cube gameplay-wise
Naturally, I perk up when my favourite topic suddenly appears in another medium
Unless youre going hard on using the Seals and other enchantments that sacrifice themselves, this card is best reserved for your deck with Tom Bombadil as its saga commander, rather than anywhere else
Its a move that gets more powerful as the game goes on, and has the potential to bring you back from the precipice of defeat