The Dark Slowking (9 / 109) card can be found in the Team Rocket Returns set.
Recently sold 5 times and based on those sales is worth around £16.73.
Currently, there are 2 open listings for this card on ebay.
This card sells in the range of £6.08 to £27.38, making it quite volatile.
This Rare Holo card was illustrated by Kouki Saitou.