Re-celling lithium batteries isn't a practical option, since there's complex electronics inside the battery as well as cells.
Changing to NiMh would lose range, plus the difficulty of making a new case to take the cells. There would also be a loss of performance since an NiMh based one would have somewhat lower voltage.
The price would be very little less at present, and quality NiMh prices are rising as they become more scarce.
The 500 charges that I know you are concerned about isn't an absolute, no-one knows exactly how many your battery might take until they have been around long enough. It is also 500 full charges, part charges counting as such.
The battery is guaranteed for two years, so for a weekday commuter that's a little over two years after allowing for holidays etc, assuming they use the entire battery content daily.
Basically, if the battery lives up to that, it's 2 pence per mile battery cost based on a 30 mile range, cheaper than any other vehicle bar a normal bike, and possibly cheaper than walking, given the poor quality of so much modern footwear.
The electricity cost per mile is far too low to express.
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