The cheapest post-paid plan is M1's Data Lite plan which is $21.40 a month, but you only get 3GB of data. If you exceed this, the maximum you will pay is $51.36.
A comparison of the existing plans is below:
Post-paid monthly subscription plans for unlimited data
StarHub, 24-month contract: $43.66
SingTel, 24-month contract: $30.50 (50GB)
SingTel, no contract: $34.50 (50GB)
M1, no contract: $39.48
(SingTel's plans are compared here because at 50GB, they are pretty much unlimited.)
Post-paid monthly subscription plans with data cap
M1, no contract: $21.40 (3GB), usage cost cap is $51.36
StarHub: $32 for 60 days of usage up to 2GB
SingTel: $12.60 for 3 days of usage, unlimited data
M1: $20 for 30 days of usage up to 1GB
Of course, the absolute cheapest option is to get on a multi-SIM plan with the carriers now, cut your SIM card down to micro-SIM size and pay about $10 a month to share your data usage with your iPhone. This involves some skill, or knowing someone with a SIM cutter. No news about whether the carriers will offer a multi-SIM option with the iPhone 4, which uses also uses a micro-SIM.
Note: All prices are accurate as of 9.30am, July 22. Please check the iMerlion website for updated prices.
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