The Arceus (DP56) card can be found in the Dp Black Star Promos set.
Recently sold 4 times and based on those sales is worth around £7.99.
Currently, there are 18 open listings for this card on ebay.
This card sells in the range of £0.65 to £15.33, making it quite stable.
This Promo card was illustrated by Ryo Ueda.