Many 3g providers now have pay-as-you-go options for mobile broadband. In my case, the provider has the following options:
Pay for 1 day up front - up to 500 MB data
1 week - 2GB
1 month - 5GB
Ideally the device would have configuration options to allow at least daily and weekly limits to be set for such devices in addition to the existing monthly limits.
Also, the balance systems allow for the start and end period to be manually defined, but for 3g failover, that start period is potentially at any day and/or time.
The Balance systems allow the USB dongle to be disconnected in software while on standby, and so there should be an option that that if the device is re-enabled for failover, any bandwidth allowance monitor is notified at that time.