The Slowpoke & Psyduck GX (218 / 236) card can be found in the Unified Minds set.
Recently sold 13 times and based on those sales is worth around £64.26.
Currently, there are 6 open listings for this card on ebay.
This card sells in the range of £48.23 to £80.29, making it quite volatile.
This Rare Ultra card was illustrated by Miki Tanaka.
The fact that it is a GX card makes it one of the more valuable cards.