Cheapest returns from NRT to TPE are approx USD$200 RTN plus taxes. Keep in mind prices vary dramatically, especially at this time of year, depending on the departure date. Delaying your departure just a day or two can result in the ticket price being halved or even less.

NW, UA, CX and a number of other Asian carriers (Eva, Air Nippon, China Air) fly this route. Check their local Japanese sites (in English) for fare information.

If you provide more specific information (departure date, preferred airline) I can do a search of agencies here for you. I usually find some amazing deals very close to departure dates.

You may also like to try Ikon Travel - my preferred agent here in Tokyo. HIS also are a popular agency for foreigners, but service can be scatty.