So far most have been replaced for free. Some even after the warranty expired. There have been several long and interesting posts on this subject. In theory the batteries should last a loooooong time. They can also be rebuilt/ refurbished.
By design the Insight does not have several key parts that often need replacement on other cars. The muffler is stainless on the Insight. There are no alternator and belt, distributer and cables, hydaulic pump, fluid and belt, and the timing is done with a steel chain not a plastic belt. Taken together the servicing and replacement of these wearing and service requiring parts on a "normal" car is likely greater than a battery pack on an Insight. The price has been coming down too.
I replaced two muffler systems, two alternators, two cable sets and distributer caps, and one timing belt on my trusty old CRX. I'm betting on the Insight being cheaper.
The Feburary, the infamous IMA light appeared, and sure enough, autostop, assist, etc, stopped working. I still managed 80mpg the next day dropping the Insight into the dealer. I strongly suspect they were in the "legal CYA mode" when they said you can't drive this car with a dead hybrid batter pack. I had nearly 95,000 miles on it.
They nearly gave me a heart attack stating the battery pack, BCM, and MCM unit would cost $6,500. They later told me Honda would cover all costs above $450. They had it repaired in nine days - very quick compared to stories I've heard.
The two things I believe brought this on was the 2-3 times I let the 12-volt battery go dead, causing the hybrid battery pack to drain trying to save it. The new software (it's a 2000 Insight) might also help prevent this. Also, I drove on the agressive side the first 2-3 years.
Being older and wiser, I expect this new battery pack to last well beyond another 100,000 miles. I have not had a single recal after three months, even driving up the Rockies on I70 west of Denver.