One of my favorite coral is GSP. When it's together as a big patch is soo nice. I think Josh has the entire bottom of his tank covered with them, I only saw pictures but it looks soo cool. It's got be a supply and demand issue. GSP is very common, and I am assuming most of us have it, and give them away for free, or for very little. But the expensive zoas (rasta comes to mind)is not as common in peoples tanks, that's why you can sell it for a lot of money. I personally think rasta is very nice zoa's. I reentered the hobby recently and so i am starting my tank, so I wouldn't want to spent a lot of money on just one rasta polyp, when for the same amount of money, i can buy 3-4 coral to fill up my tank.
Maybe we should make a poll to see what expensive coral we have. Maybe if a coral was readily available from hobbyist, vendors wouldn't sell it for a lot of money.
10 years ago, I remember any type of Acropora and SPS in general were no less then $60 for a 1" frag, now, people give them away for very little or free(of course not all acropora).