I did a bit of research on prepaid SIM's two weeks ago. Findings are in line with the thread consensus. The three least expensive are CSL (Telstra), PCCW and Peoples (China Mobile). I've used PCCW and CSL, stayed with CSL for better call quality (at least to me). Breakdown:

CSL: $0.05/min off-peak, $0.12/min peak, $2.5 per month, $0.7 inter-network sms
PCCW: $0.06/min off-peak, $0.12/min peak, $2.5 per month, $0.7 inter-network sms