I can’t seem to find the data rates for unlimited usage on the SingTel 3G $15 hi!Card. It just says it has S$18 value instead of just S$15. It also states “Pay Per Use Data at discounted rate (50% off): 0.27¢ per kb (charged in blocks of 5kb)” which means it is expensive if you are going towards data usage.
That is not really true. SingTel has a 1GB plan which cost S$7 for a period of 7 days for prepaid card users. I think that is more than enough. I hardly use more than 1GB per month on my iPhone 4. You can find most of the optional Value Added Services (VAS) for prepaid plans here.