Limiting traffic through a gateway in a failover setup

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[acecombat]

Is there any way to limit how much traffic can go through a particular gateway for a particular timeframe?
We've got setup an unlimited main gateway with a 5gb cellular gateway (Provided from an Option Globesurfer II) as a failover for when the main link is down.
Is there any way we can limit how much traffic Endian will allow out this secondary gateway (Per day, month etc.) as anything over the 5gb gives a really nasty bill!
Unfortunately the Globesurfer does not have this built into it.