Anime News Network reported a study by the Nomura Research Institute (via Reuters) on the Japanese otaku market which found that the otaku market for manga was the largest such market in Japan.  The study estimated that it consisted of around 350,000 consumers that spend around 83 billion yen ($729 million) per year on manga and manga-related products.  Note that this is only the hard-core fan portion of the Japanese manga market, whose consumers are spending over $2000 per year on their hobbies. 

Other notable otaku groups identified by the study included 160,000 videogame otaku who spend 21 billion yen per year (around $184 million), and 110,000 animation otaku who spend 20 billion yen per year (around $175 million). 

The study, which was conducted over the Internet using a sample of 10,000 Japanese citizens, identified and quantified nine other otaku groups, including celebrity otaku, computer otaku, etc.