The Mewtwo & Mew GX (SM191) card can be found in the Sm Black Star Promos set.
Recently sold 43 times and based on those sales is worth around £39.02.
Currently, there are 35 open listings for this card on ebay.
This card sells in the range of £21.22 to £56.82, making it quite volatile.
This Promo card was illustrated by sui.
The fact that it is a GX card makes it one of the more valuable cards.