Driving 75 to 80 will pull mileage down to the low to mid 40's.

As for the battery, we are already logging in excess of 200,000 miles without battery replacements on the model that I own. I've heard of only one battery replacement thus far at 207,000 miles. As far as the price, they are already dropping substantially. Last I heard was that they were in the $3k range, and they were still over $5,500 when I bought my car.

Other offsets are no alternator, clutch, torque convertor, transmission, (Prius uses a planetary gearset like a differential to put power to the wheels & the electric motors act on them to mimick a CVT), power steering pump, or starter. Regenerative braking takes stress off of the brakes, not that I'm hard on brakes with my driving style to begin with as I have over 106K on my Dodge truck, and am still on the original tires and brakes.

My goal for this car is to see 300,000 miles before I buy something new, unless something major hits the market in battery technology that will allow the car to become an electric car with an occasional engine assist. There has been some recent advances announced with Li Ion technology, using nanotechnology, that is allowing for 80% recharges in less than one minute, with zero degredation of the battery. Storage density is reportedly increased massively as well.