The Lucario (122 / 123) card can be found in the Mysterious Treasures set.
Recently sold 6 times and based on those sales is worth around £15.00.
Currently, there are 6 open listings for this card on ebay.
This card sells in the range of £9.85 to £20.15, making it quite stable.
This Rare Holo Lv X card was illustrated by Ryo Ueda.