The Dialga GX (138 / 131) card can be found in the Forbidden Light set.
Recently sold 5 times and based on those sales is worth around £16.00.
Currently, there are 17 open listings for this card on ebay.
This card sells in the range of £9.90 to £22.10, making it quite stable.
This Rare Rainbow card was illustrated by 5ban Graphics.
The fact that it is a GX card makes it one of the more valuable cards.