SingTel has sent me a message that I did not expect to come so early. The promo is going to be over. From 16th September, any extra data incurred from your data plan will be priced at S$10.70/GB (from the current S$5.35/GB) and will be capped at S$188 (instead of the current S$94).
So, for those who have not downloaded the app to monitor the data usage, I urged you to do it now. Of course, if you are confident that you will not overshoot your data usage, you need not worry.
So far, StarHub who charges higher per GB (at S$6.42/GB) mentions in their price plan that this pricing is valid till 31st December 2013. As for M1, it is still charging at S$5.35/GB until further notice.
I guess the rest might follow the similar path of SingTel. Never expect it to happen so soon (I did not even read the fine print for a start).