The last time I bought a DVD in Hong Kong, it cost $20 HK. That was not from a shop, but from a street seller and it was about two years ago. I believe the shops sell DVDs for anywhere from $40 HK on up.
Chinese prices vary. Beijing has DVDs for 9 or 10 yuan (that's roughly 9 or 10 HKD) and other cities have DVDs for different prices. My particular city (northwest China) sells DVDs for 6 yuan.
Enjoy your trip!
