You can buy a refurbished matched battery from green tec auto for $1000.
They work like a champ.
As for the mileage, we put one of those batteries in a 2010 Prius with 350,000 miles on it. Battery was dead, horrible mileage. We bought the car that was in otherwise decent shape for $1,400, changed the battery now mileage is like a new car. Needs brakes next.