The 10 year 180k warrantee is on the battery and control units for the battery. It was 8-80 then they extended it to 10-150. Then some California attorney got a judgement on odometer readings being higher than real readings, so Honda went to 180.
Source is Insight central website.
Replacement batteries were 5k, now they rebuild them and they are around 1900.
There has also been considerable improvement in Nickel Metal Hydride Batteries over the last decade.
If you think your battery pack is going bad I would contact Toyota to see if they offer and kind of consideration as far as replacement costs. You might contact Toyota directly if you have a question. I would think they would possibly follow Honda's example.
My battery pack is made up of numerous D size cells. You may find some company offering an improved pack. Some of the Insight owners have done a lot of work rejuvenating their own packs, but it is fairly complicated. Of course another pathway would be to buy one from a wreck. The problem is you need to keep the pack in a certain percentage of charge or it will go bad if totally discharged.