Mine was astronomically high in the first month of use...3 times over my 4GB limit to be specific (for a total of 7GB of data usage for the month). I thought this to be impossible, especially since in the second cycle of usage, i didn't even hit my 4GB max and used it about the same. I called AT&T a dozen times and got the run around or the "Tough Sh*t" response from them, explaining that they can only go by what the computer tells them, and it said i used that much so i must have.
Finally one day i went online and chatted with a rep and explained the situation and asked them to check out my current cycle usage...they did and noticed the drastic difference, they waived the first months overage fees and assured me that any issues that existed that caused excessive data usage was remedied. I've had no issue since, and this cycle I'm using only a hair higher than normal (because of the holidays and people messing with my phone, lol).