Before I bought my Zero SR, I did a lot of ruminating and calculating...my situation is a lot simpler since I'm a single man, but I still don't like to waste money!
My conclusion was that the bike could theoretically pay for itself in about four years, assuming I do my daily commute (which Google maps says is about 45 miles but my odometer says is 50) on the bike instead of in the car at least four days a week, and that the IRS's allowable deduction for car mileage (currently $0.569 per mile) is fairly accurate, and maintenance for the bike is much, much lower than on the car.
I have to admit to being a bit skeptical of my own analysis, though. First, although it's hard to imagine the IRS being generous in any way, it's hard for me to believe it really costs $0.569 to drive a car for one mile, even if you do get all your maintenance done by a mechanic at shop rates. Second, without the bike I'd save the insurance cost -- it's not ridiculous but I still start off each month $60 behind. I also don't have any real access to numbers as far as maintenance on the bike...I can calculate tire changes and brake work, but I don't know how much running gear maintenance will run.
The bottom line, for me, is that it's hard to make a purely financial case for an electric bike, at least for the SR. Batteries are still just too expensive, volumes are too low to get into real economies of scale, and the purchase price is still too high to make up in a reasonable amount of time. I'm near the maximum commute you'd want to do daily without the power tank or midday charging, I live in San Diego so weather is very rarely an issue for me, and the payback period for me is going to be four years, even using what may be fairly friendly numbers.
But for me, it's not ALL about the financials. First, I'm a motorcycle guy, I'm GOING to own a motorcycle until I'm unable to ride any more -- the only question is which one. Second, I'm an environmentalist and I do feel good about doing something to help the planet. Third, I'm an EE and I definitely have a toy problem, so the electric bikes have a huge appeal for me. All of this makes me okay with the concept of being an early adopter, and paying a few (not all that many) extra bucks to help the market develop.