Aside from price, you might want to go to HK to get Apple products because they’re usually available there earlier. The iPhone 5 is just coming out now in China, it’s been available in Hong Kong for 3 months, and at lower cost.
On the cost issue, how much the price varies depends on what you’re buying. Check this out:
If you buy the base model iPod Touch in China versus Hong Kong, you will spend about $50 more: roughly $240 in China to $190 in HK.
If you buy the base model 15″ Retina Macbook Pro in China versus Hong Kong you will spend about $500 more: roughly $2,650 in China versus $2,150 in HK. For a purchase like that, it’s definitely worth going to Hong Kong.
And if you’re spending any more than that (which isn’t difficult to do, by the way), it becomes fairly ludicrous to buy inside China unless you enjoy burning money.