I usually buy a 68$ buy a China Mobile (Peoples) IDD SIM card from the 7-11 after customs. 0.25HKD/min to many countries and HK (thats high for HK, but low for the rest of the world). They now charge a 2$ a month surcharge to keep it active. You cant get data on it though.

If you buy the 48$ non-IDD SIM you can get unlimited data for 9$ a day (and even cheaper if you do 3-7 day). Its the rest of the day you register + the full 24 hours of the next day, so if you charge it in the morning, you can get nearly 48 hours of data for the 9$.