Depending on the measurements being used which I am assuming 1MB/s is part of a 95th percentile measurement this is hardly equal to 320GB of Actual transfer. While results are always different when trying to determine how many GB's a 1MB/s it is no where near 320GB of transfer and much closer to 150GB with maximum being 200GB of transfer.
For example in reviewing a customer who was pushing 1.3MB/s on a 95th percentile basis the Actual GB's were right around 220GB's of transfer.
How 95th percentile actually works is every couple of minutes your server is polled, these polls are stored throughout the month. At the end of the month they are arranged from highest to lowest. The upper 5% is knocked off and the very next reading is what you are being charged for.
Some providers take that number and use a formula to let you calculate what your "Actual" GB's would be if you sustained the 95th percentile reading, this does not actually give you a true and accurate "Total" GB's, but rather a scenario what if based "Total GB's".
Other providers will actually parse your log files add up the total bytes transferred and give you the grand total. My opinion this method is what you the customer should be looking for. Utilizing the above method and giving you a lame 95th "what if" reading ends up costing you 100-150GB of transfer you never really used.