The purple passion and the mystic potion look pretty much the same. There is variation in color and pattern in every clutch of every morph, so, I'm thinking...same gene, just pick your breeder stock with desirable characteristics.
This is how I understand it, and correct me if I'm wrong, folks:
RDR held his Phantom prices pretty steady. When Anthony McCain discovered the Mystic, I think he marketed it as "unproven Phantom" in a line separate from Ralph's. After he proved them out, he kept the Mystic name rather than "McCain-line Phantom." Mystics were sold more affordably, so more breeders have them now. Some people still think that they're totally separate animals (different genes), so prices remain divided.