I think there are several reasons they don't show up often.
1) Most are not very colorful so divers don't collect them, although I have seen some pics of bright green ones in the wild.
2) It is hard to find small ones in the wild. The shipping cost on a 2-3 foot anemone is huge. For most importers its just not worth it for a brown anemone.
3) They are found a little deeper than other carpets and don't occur in clusters.
As for the price, I must admit that I got mine before the craze. My first one (18") I bought as a mertens for $25 wholesale (about $50 retail). The second one (~8") I got as an "assorted carpet" for $15 wholesale. I bought one the size of a silver dollar a year or so ago for $40 retail.
The pricing is almost a reverse supply and demand. In the general population there are not many people who would buy one for any price. They just aren't that attractive compare to other carpets. On the other hand, there are a few weirdos

who will buy one at almost any price when one actually does come in. So if you bring in one, you can sell it for $300. If you bring in 20, you can sell 3 for $300 and then you are stuck with the rest of them.