The Mewtwo & Mew GX (SM191) card can be found in the Sm Black Star Promos set.
Recently sold 7 times and based on those sales is worth around £35.00.
Currently, there are 22 open listings for this card on ebay.
This card sells in the range of £27.43 to £42.57, making it quite stable.
This Promo card was illustrated by sui.
The fact that it is a GX card makes it one of the more valuable cards.