It does seem a little excessive. The most I have ran up in a day was 80mb but this was with downloaded apps running such as one that was constantly checking the latest football scores.
By default apps, I assume you mean email, facebook, etc. If so, it depends what you would be doing with these apps, for example, if you opened emails that contained attachments or watched facebook/youtube vids then this could be the cause. If, on the other hand you have just visited a handful of text based sites then I think you need to get back to talkmobile and ask for more info about times when this data was used.
