The M Gengar EX (XY166) card can be found in the Xy Black Star Promos set.
Recently sold 23 times and based on those sales is worth around £24.68.
Currently, there are 11 open listings for this card on ebay.
This card sells in the range of £15.89 to £33.47, making it quite stable.
This Promo card was illustrated by 5ban Graphics.
The fact that it is a EX card makes it one of the more valuable cards.