Have a look at the P14 big spot in the media lounge now and the poster breeds all sorts of black rays.
Black Diamonds will go for more than Leos.
I have recently heard of a breeder paying more than £20,000 for a pair of black Rays (i think this may have been a while ago and it is third hand info via my LFS but I have also heard it from a second source)
Odd question though, why do you want to know the most expensive?
I do not think Pearls have any links to the Black Rays but willing to admit I know very little despite owning one wrong. My Pearl Male is not expected to get much past 12 to 15" but I expect some females will. The Pearls can range from £500 ish for Pups, maybe less but it does come down to the quality and source i.e. Captive or Wild caught. My Pearl is around 10 inches at around 2 yrs old.
I know of a pair of Pearls up for £1200 for the pair of pups (Captive Bread and very pretty)