The Slowpoke & Psyduck GX (218 / 236) card can be found in the Unified Minds set.
Recently sold 7 times and based on those sales is worth around £39.85.
Currently, there are 4 open listings for this card on ebay.
This card sells in the range of £21.92 to £57.78, 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.