The Cool Porygon (15 / 53) card can be found in the Wizards Black Star Promos set.
Recently sold 30 times and based on those sales is worth around £24.40.
Currently, there are 9 open listings for this card on ebay.
This card sells in the range of £16.13 to £32.67, making it quite stable.
This Promo card was illustrated by Hiromichi Sugiyama.