Upvote:2
Vanilla Air lists a fare to Taipei from Tokyo of Β₯6990 (roughly $60 USD).
Most of the low cost carriers (lcc) in Japan maintain their own ticketing infrastructure, and tickets are hard to find outside of that. The carrier to choose for the Tokyo->China leg would depend on the city you're headed to in China; and, if you wanted to leave from Tokyo or another city. The same site has Hong Kong fares from Β₯8890 ($80 USD); but, if you want to go to the mainland, you'd need to find a different carrier.
(edited to add) Vanilla Air has some not-so-flattering reviews, so, caveat emptor and all that. The linked site has some good English information on all of the LCC in Japan.