I will assume you have an iPad, check out the app SkyScanner -- searches for the best deals around and hasn't failed me yet.
But the above user is correct, it is always cheaper to travel domestically in China. Hong Kong is considered an international destination. So, if you can spare the time, a short train ride to either Shenzhen or Guangzhou and then a flight from those cities can often save you hundreds of dollars. Yes, hundreds.
Another option is a train from HK to Shanghai which will set you back US$62 on a hard sleeper and takes around 20 hours. Check out seat61.com for more info.
Safe travels! :) over 7 years ago