There are NO additional regular maintenance costs to be expected. Change oil & fluids on the same schedule as any other Accord. Nada. Nothing. Zip.

Honda has replaced "single digits" numbers of battery packs from Insights. That's from one of their execs a few months back. Bottom line is that the batteries are very reliable. The warrantee in the US is is 8/80, 10/150 for the PZEV cars in the PZEV states (CA, MA, etc). The few folks here who have asked about battery replacements have all quoted somewhere around $2-3K US, IIRC. Expensive, but in line with other critical parts- tranny comes to mind. It should be quite cheaper by the time one can expect a pack to need replacement. Honda has been replacing some packs free, as 'goodwill' even if they are technically out of warrantee.

Another item of interest...
The Honda system does not require the large battery to be functional in order to be able to drive the car. As long as it can still conduct current to pass charge from the motor/generator through to the 12v battery, you will have full functionality. Just no IMA assist, and no autostop. Hondas have a 12v starter as backup. It will simply act as a 'regular' Accord. Cylinder shutdown might still work- I don't know if anybody's asked that to Honda yet. In which case you would still be able to get 40+mpg with ease on the highway.

Bottom line- replacing the battery should be pretty low on the list o' concerns, because its not a concern. Its a Honda.