if you are using ios 7, then you will know which app is eating your data plan.
disable background refresh of facebook, flickr.
they will sync your photo if you have allowed it before.
singtel will charge on per 20KB or 50KB block.
e.g. you send one iMessage or whatsapp. then phone disconnects from internet..
this is just 160 bytes.
however, Singtel will make that as 20000bytes (20KB). check previous post where singtel bill shows dozens of 20KB data usage.
my suggestion:
1. use ios7 and check the built-in data counter.
all data counter that you download in app store are useless.
2. run some chat program on the background. e.g. whatsapp, viber, skype. yahoo messenger. this will somehow keeps your internet connected and doesnt disconnect always.. lessening the 20KB block charge..
btw, i have moved to starhub because of the 20KB block charge of singtel