wow, will you look at that. there is a note on the 2005 US Brochure on the Specifications page that does say:
3. Hybrid vehicle battery expected life is 150,000 miles based on
laboratory bench testing.
see:
http://www.toyota.com/toyota/vehicles/2 ... ochure.jsp
(view either
http://www.toyota.com/images/vehicles/e ... rispec.pdf or
http://www.toyota.com/images/vehicles/e ... ebroch.pdf )
Which is odd, as in the marketing in most other countries the battery life is listed to be the life of the car.
Anyhow, the 2005 US Prius' battery pack is included in the Hybrid Vehicle System warranty, which is 8 years/100,000 miles. The battery pack is further covered by the California Emissions warranty (if you live in CA, ME, MA, NY, VT) out to 10 years/150,000 miles.
(Remember, there is a very good battery management system in the Prius. Not too hot, not too cold. No full charge/discharge cycles. All to give a long life to the battery. You rarely find such management on consumer electronics, where the goal is to sell you new batteries...)
I'll disregard the original 1998-2000 Japanese Prius. (Rumors of a free trade-in program for these batteries as the management system wasn't the greatest (could deplete the battery).)
As for the Classic Prius, there are many owners over 100,000 miles now (I'm no where near one of them.), plus there's the famous taxis. I've heard of a few battery packs being replaced by Toyota just for real-world extreme climate testing purposes. There was one because of a screw drilled into it by an audio installer (about $6000 for a complete new pack, brackets, and labor costs), and then there was one Classic owner on this site a while back (I don't have the thread, I think it got lost in the last database crash) that was experiencing degraded performance and needed a new hybrid battery (they got a used pack <$1000, and got the $200 bounty from Toyota for the old pack).
I can't say how the new pack on the 2004+ Prius is, but I can only assume that they're getting better.
Supposedly you can replace individual bad cells, but Toyota USA seems to suggest just a whole pack replacement.
Pricing will depend if you're going for a new unit from the dealer (and any discounts you can get from the parts department with coupons and such), or if you go to a junkyard/scrapyard and get one from a wrecked Prius. Used battery pack pricing on eBay usually goes for < US$1000.