I'm constantly surprised that noone seems to mention Spring Airlines on Asian travel forums. They have easily the cheapest Chinese flights, and I always check their website first when booking domestically. Srsly, they fly Shanghai - Hong Kong for about 700 RMB return, all tax etc included!
Granted, their seats are quite small and they do a funny sales thing onboard the flights - but it's more than compensated by a pair of headphones and the price of the ticket.
I'm guessing it's because they don't want to partner with cTrip & expedia -- I can't find Spring tickets there at all?? You have to set up an account at www.springairlines.com and buy direct instead. Hey, as long as the prices are dirt cheap and the planes are safe, works for me!