We're around 10 days in, and we've had enough time now to form some general observations about the cost.
On the highway, as a pure hybrid, the volt gets around 32 mi/gal. Not bad, and that is a worst-case scenario for a hybrid. This really only impacts us when we go on a long trip, but it is almost 10 mi/gal better than the van, and 5 mi/gal better than the old Passat.
As a pure EV, we seem to be getting just under 2.2 mi/kW-hr. That's spectacularly bad for an EV. The Fit EV (which we have dubbed the Blueberry in a nod to Psych) gets 4 even with my lead foot driving style. Tesla claims 3 for the Model S - even with its ginormous battery. 2.2 mi/kW-hr is just over 70 MPGe.
Chevrolet, in their informational data shown either on the web or in-car, shows a downright obsession with MPG - which they calculate by dividing the odometer reading by the amount of gasoline consumed. They do keep a cumulative kW-hr intake that you can view on a charging detail page, but the prominent placement of MPG seems intended to fool you into believing that electric miles are somehow free.
We live in Santa Clara. We have perhaps the lowest electric rates in California. Even so, the top tier is 10.2 ¢/kW-hr, which is the equivalent of $3.41 per gallon in energy equivalent terms. That's cheap, yes, but hardly "free." Now, granted, some charging stations - like the ones we have at work - are free, but this is going to be the wife's car, and she's likely to do the overwhelming majority of her charging at home.
So what are the final numbers? In terms of money per mile, the Fit EV is 2.55 ¢/mi. The Volt as a pure EV is 4.63 ¢/mi. On the highway, as a hybrid, with $4/gal gas, it's 12.5 ¢/mi. Of course, that's purely the cost of energy, and it assumes all kW-hr are 10.2 ¢ and all gas is $4/gal.
Clearly, "don't believe the hype" would be overstating it. As an EV, the Volt is almost 3 times less expensive per mile than it would be as a hybrid. But I do think that Chevy's pushing straight MPG is a bit disingenuous.