Target battery capacity is 16kw. Electricity costs range from ~6.7c/kwh (Idaho) to ~30c/kwh (Hawaii). The National average is about 11c/kwh.
If you were to discharge the battery completely every single day, and fully recharge it every single night, the prices would look like so:
Idaho: $1.07 per charge, $7.50 a week, $32.10 per month.
Average: $1.76 per charge, $12.32 a week, $52.80 per month.
Hawaii: $4.80 per charge, $33.60 a week, $144 per month.
*Again those numbers are considering you drive 40 miles, 7 seven days a week.*
Using today's average US gasoline cost of $1.67 for reg unleaded, the volt would dollar/mile be equivalent to 62mpg in Idaho, 38mpg national, and a not so attractive looking 14mpg in Hawaii.
That is of course not factoring in any nightly discounts for off peak usage, and it is also comparing it to rather low cost gas. If gas were to find it's way back up to 4 bucks a gallon, those "mpg" numbers would be more like 150, 91, and 33, respectively.