Most of the wear parts like tires, brakes, and shocks cost about the same as most small cars. All the regular maintenance costs the same as any other honda. Replacing the clutch or transmission costs about the same as other honda's as well.
The battery pack costs $3,600 Canadian at my Canadian Honda dealer (I just asked last week out of curiosity). But the battery pack should last the life of the car under normal use or at least 10 or 15 years. The Insight battery pack has a 8 year warranty. Most cars have a 4 year 60,000 mile warranty on their gasoline engines and we know most cars don't have engine problems for 3X longer then that.
Considering that i'm saving $1,500/year on fuel costs i'm not worried about the battery pack.