I would really be interested to see you breakdown how gasoline savings could make up a $15-20K gap. I would really, really be interested to see you break that down for me.
Let's use rough numbers since everybody's circumstances are different:
Say it costs $30 per week to fill the car over the lifetime of the payback (at $2.50/gallon, that is what it costs me). That is $120/month...$1440 per year. If you only need to use gas with the volt once every couple of months on average, assuming you only use it to go to the store, work, etc, that is only 6 fill-ups a year, or $180. If you are not using any gas, the price is ovbiously $0.
$1440-$180 = $1260/year savings
$15K/$1260 = 11.9 Years
$20K/$1260 = 15.8 Years
I understand this doesn't assume you already drive a hybrid, but even at half the cost per year you would experience with a hybrid, the Volt is still at least competitive, especially if you are
never buying gas. Also, this does not factor in the cost associated with charging the battery out of an outlet of your home. However, it also doesn't factor in things you won't need to do with an electric engine that you would with a traditional or hybrid engine system...oil changes, transmission repair/replace, etc. Furhtermore, this doesn't take into account volatile oil prices...which would greatly accelerate the payback if/when oil prices rise. If all your assumptions against and all assumptions for equal out, you are still looking at roughly a 10 year buy back.
Is it a good car for the guy that commutes an hour everyday for work? No. Is it good for the guy that commutes 10 minutes to work everyday and would only need a partial charge every night? Maybe, depending on all the factors associated.
Nevertheless, if Honda or Toyota can create a plug-in hybrid that doesn't use gas to power a drivetrain, and can come up with some exotic battery technology that would reduce the price premium for a plug-in battery system, then of course the Volt is more expensive.
What the Volt really offers here is increased efficiency by using gas to charge a battery, as opposed to driving a drivetrain.
Personally, I say both the hybrids and plug-in models are not worth the price. If you are getting 40 mpg with a traditional Honda Civic for ~$15K, that is about the best there is in an economic sense.