The Volt battery pack is est. 16kWh capacity, but it is only charged to 85% full and is discharged to 30% SoC approx, before the engine cuts in. When the vehicle is plugged into a charger the battery SoC is restored to 85%. Hence the battery has an effective capacity in use of 8.8 kWh.
This taken from Wikipedia.

I'll let you sharp mathematicians figure out the cost of a charge.