The Cacturne ex (91 / 106) card can be found in the Emerald set.
Recently sold 3 times and based on those sales is worth around £18.00.
Currently, there are 3 open listings for this card on ebay.
This card sells in the range of £8.11 to £27.89, making it quite stable.
This Rare Holo Ex card was illustrated by Hikaru Koike.
The fact that it is a EX card makes it one of the more valuable cards.