I have NEVER seen anyone pay for an HV battery in a Prius/Highlander/Camry and it has been many years.
The accessory battery is like any other battery. It is affected by heat/humidity/age/wear and tear. They usually last 2-3 years, some more some less. They don't last that long out here in the LV heat. 2-yrs tops.
If you know someone, you can find that battery cheaper than most dealers .