They're expensive because in the wild, they are not that common. They don't grow very fast unless fed.
One of the main reasons for the high price is the colors that they come in.
Just an FYI for anyone wanting one of them, they're very easy to frag, and as long as you feed them and keep your parameters good, they'll grow into a healthy new scoly in about 9 months.
I'm actually getting ready to splurge on a Bleeding Apple and I will probably frag it in half. One of the stores near me frags his quite often. Since people aren't willing to pay the full price for a full one, but will spend $80 for a quarter of one and let it grow, he can usually come out on top if he frags and sells.