Below extracted from Maxis website today (29.3.2013)
*Any additional usage will be charged at RM0.10/10KB.
** You will receive an SMS notification once you've reached 80% and 100% of your monthly allocation. Once you've reached 100% of your monthly Internet allocation, subsequent usage will be charged at RM0.01/10KB. For mobile internet plans with quota of 1GB and above, you may continue browsing at a reduced speed for FREE. However, to continue browsing at regular speed, we recommend that you purchase a quota upgrade by dialling *136*9#.
I remember earlier the chares for additional usage changed to RM0.30 / MB and now back to RM0.10 / 10KB and FREE for quota 1GB and above.
Good luck for those subscribe below 1GB.... and those subscribe Time Passes.