Generally the SH route is the most overpriced of all the routes ex-TPE. The taxes and fees alone are 100$ about. Often the cheapest way to get to China from TW is to fly Cebu Pacific through Manila. However, there are other options....

If you're leaving from mainland China to TPE for a R/T and have a Chinese bank account you can purchase a special from the CZ specials site here:

http://air.tenpay.com/pages/international/tejia.php

These flights are only about 1000rmb, but none of them go to Shanghai....of course you can just link up with a high speed train or a connecting flight for under 100$. Wuhan is an option for that....if you play around with dates to shanghai you'll find a round trip for about 2000 rmb which is about 300$. Again these don't work ex-TPE but only ex-PVG or any other city which CZ has flights to TPE from. It's the cheapest option I know of....

CTrip does not price well for international flights - check out qunar.com or look at the specials on the airline pages and wait a little bit longer....Chinese flights are rarely cheaper months in advance....Usually I just use Asia Miles since it's a good deal compared to actually buying the ticket....