At what mileage point do you have to replace the batteries on a Prius? What do they cost to replace? That may be why the used Prius are so cheap. (I don’t know).
Also, will the second set of batteries last as long as the first?
I drive a Miata, so a Prius is not a realistic choice for me.
They are warranteed for 100,000 miles or 8 years. In California, it's 150,000 miles or 10 years.
In 2014, the cost was between $2,300 and $2,500 to replace the full pack, depending on the model. So, that's a maximum of 2.6 cents/mile. But, it doesn't include labor.
However, a full battery pack doesn't necessarily need to be replaced. It's made up of 28 modules, and each can be tested individually and replaced as necessary.
http://www.thehybridshop.com/media/blogs/toyota-prius-hybrid-car-battery-prices-replacement-costs/