Hybrid-related components for hybrid vehicles are covered for 8 years/100,000 miles. The HV battery may have longer coverage under emissions warranty. Refer to applicable Owner's Warranty Information booklet for details.
Non-warranty, Prius battery replacements have been running around $4.7k including labor. However, some have used salvage packs, $500-1,200 and I'm working on a battery refurbishment technique that should be significantly less expensive, $50 although you'll have to do a lot of labor, ~40 hrs.
4.7k! eek... This was the determining factor on why I decided on the Jetta over a HCH. The honda salesman wouldnt even quote me a figure for battery replacement, so I knew it was a pretty penny.
This is perfectly fine because people have different thresholds of 'risk avoidance.' An engineer, I enjoy dealing with new technologies. Others, like my wife, are not technical and really need a system that minimizes her need to master new technologies.
from what i have seen, the incidence of needing a replacement battery has been very small. there is a well known prius that recently reached almost 350,000 miles of use, and never had a battery problem.
unfortunately, it was hit by someone who ran a light and pretty much totalled. the driver was fine, the poor car wasn't.