(Caveat) Just my experience...
From October 2004-present (35,000 - 53,000 miles), the only repair I have had done are new front brake pads and rotors. (I also had a 42,500? mile tune-up done, which in hindsight I could have done most of it myself after getting to know the car better.) The tune-up and brakes/pads cost a little over $600.
Do a search on this forum and you will find out the Insight does not have a lot of the parts other cars do (examples: alternator and spark plug cables). This is a hidden maintenance savings people often forget.
I think I am due for a new 12V battery ($70-100, or $30ish if you go with an oversized Civic battery).
Remember that the main battery pack is still under manufacturer warranty for 7 years, 100k miles [Mod correction the original IMA warranty is 8 years / 80K miles]. And if your mileage exceeds that Honda does a lot of goodwill extensions if it fails (although expect the worst, just in case). I have a hunch Honda will extend this 10 year, 150K in all states soon (see US IMA Warranty Extension under General Discussion forum). I just think it has not been done yet due to financial and tactical issues on Honda's part.
Keep in mind that if you are buying the Insight solely to save $$$ on gas, look for another car. Most Insighters seem to have very few maintenance issues with their cars, but remember the 80s movie Money Pit? It could happen, and you have to be ready for that when buying a used car. Good luck.