There are plenty of stalls along Apliu Street selling 5-day China data SIM cards for $50-60 HKD. Just tell them data SIM and how many days you need and the seller will show you the option and price. Prices are typically marked on the pile of cards already. I recall it's just pop in and it activates on its own. No registration of ID whatsoever and the card breaks through the Chinese firewall as they are HK SIMs. Happy Telecom cards are a bit more complicated so stay away from those.