I was born to reply to this.
Cost per mile for a vehicle is a highly variable number. If you do your own basic maintenance, own inexpensive cars, drive em' into the ground, and have normal mileage, your costs could be as low as 30 cents a mile. On the other hand, the IRS and most business allow around 50 cents a mile for use of a private vehicle. If you drive a Ferrari, use gold plated oil filters, and get a new car every couple of months, your cost per mile will top $1.00 a mile. There are also calculators that factor in the "cost" of environmental and health costs (although the user does not incur this cost) which will give you a figure over $1.00 a mile.
As an engineer, I calculate the pay-back on every freaking thing. Needless to say, it drives my wife nuts. I am mentally using the figure of 50 cents per mile, even though it is probably closer to the 30 cents per mile figure. I use my actual bike odometer reading to calculate the savings, since I very seldom bike solely for pleasure. I spent $2000 to electrify my Xtracycle with a Stokemonkey set up. I spend about 3 cents a day on electricity (I have one of the devices you plug things into and it monitors your electricity use, so I know this number pretty accurately). You can pretty much ignore that cost as trivial. The payback period was therefore around 4000 miles, which I hit in the late fall. Now that also ignores bike maintenance and parts, which haven't been much so it is probably not a bad first guess.
In conclusion, even a relatively expensive conversion has been paid back and it took just under two years. I am a pretty serious rider and commute on the ebike most days. My commute is around 14 miles, round trip.
So yes, ebiking pays off, but you have to be pretty dedicated to doing it instead of driving. That means I carry every kind of weather protection gear and have often ridden in the rain, very cold, etc.
As to why more people don't do it, I have studied that quite a lot actually. The reasons are not surprising. First of all, most people feel the weather protection and safety of a car (whether perceived or real is another matter) outweigh the cost benefits by a huge margin. My mantra is that the only solution I can see to petroleum addiction for transportation is wicked high gas prices. The news channels are getting their panties in a twist over the prospect of shockingly high gas prices at "over $4 a gallon (gasp)". I think gas prices are going to have to get north of $5.00 a gallon before people even start to think about getting a smaller car, much less an ebike.
My wife is a perfect case in point. We go to the exact same place every day to work. I ebike and she drives (usually at different times though so we can't carpool easily). Her reasons are simple: she doesn't like riding a bike in traffic, doesn't want to get wet in the rain, and doesn't want to mess her hair up with a helmet. And she is no prima dona, a pretty typical US citizen I think.
My view, worth about what you paid...
