Vault On-Demand account users are charged $5.95 per month for 5 GB of data usage, and $0.40/GB for any usage over 5 GB.
One’s monthly usage is calculated using a "weighted average formula." This means that you are not simply charged for the amount of data in your account on just one particular day and time - instead, we measure your usage many times throughout the month and calculate the weighted average accordingly. This is why there is sometimes a discrepancy between the usage in your account at the time your charge occurs and the overage you are charged and why the calculation most accurately reflects your activity.
For an example of how the calculation is performed, please take a look in the comments below.