There is actually another coed international school in Tokyo, KAIS International School, with both a junior high and high school program. It's a pretty unique type of school, a LOT smaller than ASIJ and the big ones, but it has a really good reputation for its personal bent on academics and creative programs. My friend's son went there and absolutely loved it.
Website here -
KAIS International School in Tokyo
Also, in regards to the tuition discussion, international schools are indeed pretty expensive, although some, such as Kobe international, have reduced rates for families who aren't sponsored by companies. Also, some of the smaller schools like KAIS offer scholarships to some students. It never hurts to ask.