Well, when Nissan ran the math out on it, it came out to: A 100-mile "fill-up" takes 24 kWh of electricity, or about $2.40 at 10¢/kWh. In comparison, going 100 miles in a 50 MPG Prius costs $5.60 at $2.80/gallon, or 2.3x more. Assuming the Z gets 25 mpg to keep the math easy, it's 4.6 times cheaper to run the Leaf.
Of course, the Z is about 10 times more fun to drive, and that's the factor that I would have a hard time with.
