From FAQ, bandwidth allocated for an instance is counted hourly-basis.
However on "instance overview" and "usage graphs" page, allocated bandwidth always shows max monthly bandwidth (1000GB for $5 instance) that cannot be utilized if one start the instance in the middle of month. ( please correct me if something wrong...)
To allow users to estimate actual bandwidth allocation and overage fee, these stats should display current hourly allocated bandwidth and possibly max allocation for the month.
30 GB of 1000 GB (5%)
30 GB of 15 GB (200%)
(bandwidth allocation will be 300GB in the end of this month)