There was a battery rebuilder for Honda (is it Bumble Bee?) that rebuild their pack with new cells for $2k, $6k for a Prius doesn't make sense at all unless it is loaded with profits at dealer markup.
But to be honest, Prius drivetrain is pretty reliable and $2k at 200k is not bad, with or without the fuel saving. I'd say I'd trust this more than many automatic transmission or European cars' power train, to be honest.