I have flown many times to HK, and I always compare costs with using www.expedia.com (.ca) and by using local travel agencies.

If using expedia, be sure to use different dates as it will make a difference in price. Also the earlier you book the better.

 Local travel agencies have rates that will have just flight, or with hotel. It is worth to check out for sure, as the price may be similar and it can save you some trouble.

Also, flying into Shenzhen instead of HK is sometimes cheaper too, then you can take a bus right  from the airport right into downtown HK for like 100RMB. The bus will stop at the border to do processing then you go back on the bus.

Have a great trip.

If you are flying there from inside China then use http://english.ctrip.com/

You can probably get a better fare using the Chinese version.

If not from China I religiously use http://www.hipmunk.com/

Check all the scenarios' if flying from China as JungleLife said...flying into Shenzhen may save some money. 

I've found all flights in/out of Hong Kong to be overpriced.  It's so much cheaper to fly to Guangzhou / Shenzhen and take a boat or bus to Hong Kong.  

I always take a boat to Hong Kong from Foshan. Faster and more comfortable than bus.  I have heard there is a new subway/metro opening up between Hong Kong and Guangzhou/Shenzhen.

As mentioned above if you are already in China then it would be cheaper to fly to Shenzhen and take a bus to Hong Kong. Flights from mainland China to Hong Kong are considered international flights, so they are more expensive.

first off ..why would you want to go to Hong Kong?   and then,,,I have come and gone from guangdong 6 times in the past couple years and have used Orbitz three times, expedia once, ctrip once and direct with china southern airlines once.......  all about timing in booking the flights ..........  they change a lot....  not just here in china, everywhere