Using $3.50 per gallon (San Diego price today) and 44 mpg Prius:
Avg of 180 gal/year savings over a Corolla and 284 over a Camry comes to 232 gal/year, or $812 per year, much more than the orig calc of [ ...call it 200 gallons or $450/year — $2800 present value over the predicted 8 year life of the battery.]

Multiplying the 8 year Present value of $2800 by x 812/450 = $5052. And if the battery lasts 12 years then the present value rises to $6761.