Comparison
If the owner of a Volt makes lots of long trips, their cost can approach $1.60 per 25 miles, which is more than that of a Prius, at $1.29.
If they never run down the battery, their cost is $1.01.
Which approach is better depends on expected usage.
The first x miles of the trip will be at $1.01/25 mi, and $1.60 after that. It starts cheaper, but there's a point at which it gets more expensive than the Prius.
(Assumptions from the page: gas is $2.69/gal, electricity $0.13/kWh. Local rates will affect the crossover point as well.)