The growth rate in some corals is a huge factor, but most it's color, and in some it's both. I know a local guy up here in Tallahassee who waits almost a year in line and pays around $250 for 1" frags of deep water purple acroporas. They're known to only grow about an inch a year or something crazy like that, and they're very rare because of that, and their beauty.
It's all an economic process. In the Maxi-Mod thread there was some discussion about competition with Tunze for the high flow powerheads driving prices down. That's a great idea, however if that happens, then remember that Tunze will be less likely to produce enough for people to easily purchase one just anywhere, or they could also stop innovating, or just innovate something else which is just as great a leap ahead of the competition as their current powerheads are, and make it much more complex so it cannot be duplicated as easily or even emulated as easily. I hope some of that makes sense.
Either way, if a coral is outside your budget, don't buy it. Don't complain, save up your money, sell some frags, whatever. I know that I sold over $125 in frags in 3 days over the Thanksgiving break, of NOTHING perticularly rare. Just some standard montis, xenia, shrooms, and button polyps. I saved that money to go towards paying my father back for our new 2x250w halides over our 120g new tank.