Depends on the equipment. For lenses, the difference might be as big as 20+%. For DSLRs, you might want to think long and hard since most bodies will only have country warranty. For lenses (e.g. Canon and Nikon, they usually have international warranty) which means Singapore service centres will honor the Hong Kong purchased lenses' warranty.
Yes. It is about 10-20% cheaper. I bought my 30D + grip + sigma 18-200 + 430ex in mid 2006 at about 2.6K+ with international warranty and FOC a canon bag worth SGD $80 + UV filter + cleaning kit + omibounce etc. At that time with this combination purchase in Singapore would cost around 3.2K+.
I am going to Hong Kong to attend a wedding banquet early next month and while I am there, it is time for me to get a couple of Canon lenses. The way I see it, the savings on the lenses pretty much pay for the airfare already. I am looking at 70-200mm f/4L IS, 50mm f/1.4 and 85mm f/1.8 and perhaps the 580EX II.