New traction batteries for Prius were always around $3K new - and you could get them cheaper from damaged cars. Reports of Gen. II Priuses asking for a battery are extremely rare, on par with the usual rate of manufacturing defects in other components. Today $10K will buy you a 5-6 years old Prius.
They did, however, need some other part I think of as costing around $400, but cost them $1200 for the Prius.
This is actually normal for many Japanese cars. If you need a bolt or a bracket or a bumper you can have them made locally, out of pure gold, for less money. One of my coworkers, owner of some Japanese sports car, hit something in an underground parking and needed a new bumper with hardware. That, with labor, was around $4K. So on one hand parts are expensive; on the other hand, you probably won't need them :-)
We had a Celica years ago, very little problem every. Mercifully, we didn’t get into replacement parts. On cost of Prius battery, just repeated what one owner told me, I will NEVER know.