The sedan is cheaper simply because it is cheaper to start off with (when buying new from the dealer - around 10,000JPY difference). The coupe also has more HP than the sedan.
Apart from that I don't really know why - same as BMW 3 series coupes are more expensive than their sedan siblings even though the engine is the same.
It is rare because it is fairly expensive and most people would instead either get a used Euro if they wanted luxury or if they wanted performance car, something like a GTR. Compliance has been available for over a year now, yet I still hardly see any around.