Keep an eye out for specials. My friend and I got a good deal. Malaysia Airlines are the cheapest because it usually involves a stop over in Kuala Lumpur. Direct to Tokyo with Japan Airlines or Qantas is about $1800 - $1900, we got ours for about $1200 with Malaysia Airlines. Still not cheap but we are staying in backpackers to make up some of the money spent on airfares.
I'm not sure if there are many tickets left- I think the first night at Tokyo Dome is sold out, but not sure on the others.