Id be more worried about that Distichodus sexfasciatus than what the ray is,these African Characins can be real nasty pieces of work......that for me is one risk i wouldnt be prepared to take!
Usually Castexi are around 8" when offered for sale. They tend to be larger size at birth. A smaller one could run anywhere from $250-$400 depending on pattern. Without better pics I am still gonna say peru flower or sp. peru. Here are some peru flower pics for comparison.