Battery replacement cost can be mitigated as more hybrid/EV vehicles come into production and push down the cost of batteries.
A price drop would be welcome, but $4k average price is representative looking at the Prius growth/cost reference over time.
Worldwide cumulative Prius sales are significantly larger than the Insight: 1 million by May 2008, 2 million by Sep 2010, 3 million by Jun 2013, 6.1 million by Jan 2017. Even with this volume of vehicles/parts, the average price of battery replacement in the Toyota Prius (and other hybrid sedans) is ~$4.2k.
2001-2003 Toyota Prius (1st generation) - $3,649
2004-2009 Toyota Prius (2nd generation) - $3,939
2010-2011 Toyota Prius (3rd generation) - $4,080
2012-2015 Toyota Prius Liftback - $3,939
2012-2016 Toyota Prius V - $3,939
2012-2016 Toyota Prius C - $3,807
2007-2011 Toyota Camry Hybrid - $4,892
2012-2015 Toyota Camry Hybrid - $4,892
2013-2015 Toyota Avalon Hybrid - $4,892
2006-2010 Toyota Highlander Hybrid - $6,198
2011-2015 Toyota Highlander Hybrid - $6,353
On the plus side, there is a "core credit" of ~$1k that you get if/when the battery pack is recycled.