In california where you are, the hybrid battery warranty for the Prius is 10 years / 150,000 miles.
If it fails before then through no fault of your own (i.e. you didn't do anything stupid like crash the car or damage it by tampering with the hybrid system), the replacement cost is FREE during the warranty period. That's right, $0. Zip. Zero. Nada.
In cities like Vancouver, San Antonio, New York, Winnepeg and Boston, they have been using the Prius as a taxi cabs for years. Many have exceeded 200,000 miles with ZERO battery issues.
I own an '07, and with that kind of warranty covering it, as far as I'm concerned the battery is a non-issue.
Just make sure the car has a complete set of maintenance records and that it has never been in an accident. If these two criteria are not met, don't buy the car.
Oh, and since you live in California, if you are able to find a used Prius with the HOV lane stickers, those are worth their weight in gold.
