warranty is 8-10 years (depending on a state), so few people actually pay for those out of pocket. i've heard $3000-5000 for brand new OEM (i know, huge range depending who installs it) and about $1000-2000 for aftermarket/reconditioned/used. I'm still under warranty, so no fear, even if it failed after, would open it up, clean the terminals (it's sometimes the only problem) and try to replace bad cells first (avilable on ebay at $50 IIRC). Some people also swear by equalizing charge fixing the problem, but those chargers are $300.
edit: i just saw your signature, sadly one of those many prius bashing threads on BITOG, i guess because you are bored. sorry i replied.
BTW, i saved more in gas than the "big battery" cost. Also not a single problem in 85000 miles. Most reliable car of my life (toyota yaris is the next in that distinction.