okay so I exceed my monthly data limit and is informed that ATT's gonna charge me $15 for additional 200MB (I have the minimum data plan), and my question is whether ATT charges the fee per additional 1MB or just the $15 as long as I don't exceed it again?
The $15 is for another 200MB. If you use more than that this month, you will be charged another $15 for another additional 200MB. This will continue until your monthly billing cycle restarts.
The best thing for you to do now is to switch to the DataPro plan for $25, and back date it to the beginning of this billing cycle in your OLAM. As it is now, you are spending at least $30 for at least 400 MB. You save $5 with my suggestion, and probably won't go over again this month, based on your usage. You can switch back to DataPlus for your next billing cycle.