I'm also going to Hong Kong and China this summer. The price difference is significant between flying out of Hong Kong and other nearby cities. Guangzhou - 2 hr train away - is a lot cheaper and Shenzhen - just across Chinese border is also cheaper. What I'm planning on doing is checking out Macau - it's a 45 minute - 1 hour boat ride from Hong Kong. Macau is a Portuguese colony until later this year and then crossing into China from there. The border town there - I don't know the name of it, but will post after consulting my Lonely Planet - is a whole lot cheaper to fly out of. Hope this helps - just thought you might want to know about some of your alternatives.