The Registeel ex (100 / 106) card can be found in the Emerald set.
Recently sold 2 times and based on those sales is worth around £28.21.
Currently, there are 4 open listings for this card on ebay.
This card sells in the range of £23.00 to £33.42, 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.