Forgive me for using US measures, but bear with me...
The Volt gets 40 miles per US gallon when running on petrol - which is probably only a bit more than we can expect a modern petrol-driven car to be. Since petrol is approximately $4.00 per-gallon that's $0.10 per mile.
Charging at home my electricity rates are $0.0625 per kWh. If I estimate 4 miles per kWh then when running on electric the Volt costs me $0.0156 per-mile, or roughly 6.5 times less than when on petrol.
However, if I'm not charging at home and I'm paying for public charging, then things change a bit because public charging rates are much more than charging at home. If the cost of the electricity increases by 6.5 times then it's a wash. So if I have to pay for a fee for public charging that exceeds $0.40 per kWh, then I'm actually paying more than if I were to use petrol.
The Volt charges slowly, and it really isn't intended to be used with en-route fee-based charging, but when I do use public charging Blink network will charge network members $1.00 for whatever time they can spend charging. After an hour and a half of charging I usually have a 10 or 15-mile charge... approximately 3 kWh. At that point I'm at about $0.30 per kWh. Blink charges non-members $2.00... and at that point it's clearly more-expensive than using petrol.
Then if you factor-in the reality that EVs cost more than traditional cars as
@smartie said, you have a tough time justifying an EV purchase based on cost-savings.
So for those of us who do not need to pay for public charging there is a real long-term monetary savings to be had by purchasing and using electric vehicle transportation instead of petrol when a substantial amount of mileage is considered. However, if the EV driver has to pay for public charging, then the monetary savings approaches zero or is even more-expensive on the EV-side.
I think that there are far superior reasons to purchasing and using an EV other than monetary savings. Convenience of home charging is a big one for me. I will eventually purchase another EV (for my wife) for that sole reason.