Your information is way out of date. List price for a new battery on the '04-06 Prius is about $2,900 (I don't recall exact amount, but it's under $3k), plus an hour or so of labor to install it.

However, Toyota expects the battery to last the lifetime of the car, and there's no evidence yet that they're wrong. The Prius was first introduced in Japan in fall 1997, so if there were a pattern of battery failure we'd have heard about it by now.