The Cobalion EX (133 / 135) card can be found in the Plasma Storm set.
Recently sold 5 times and based on those sales is worth around £14.50.
Currently, there are 2 open listings for this card on ebay.
This card sells in the range of £10.15 to £18.85, making it stable.
This Rare Ultra card was illustrated by Ryo Ueda.
The fact that it is a EX card makes it one of the more valuable cards.