You did not include 2 important information: which state you live in and how old the Prius.
If you live in California, the warranty for Prius battery is 150,000 miles or 10 years. In other states, it's 100,000 miles or 8 years. During the warranty period, it is replaced free of charge if it gives out.
Out of warranty, the price of a replacement battery depends on which year model. A 2001 to 2003 battery cost around $2,300. The 2004 to 2009 go for around $2,600. I don't know the pricing for the later models. Also, the price does not include labor which can run about $300 to $500 depending on where you live.