I think it's worth mentioning another point besides gasoline and maintenance costs: overall longevity of a Model S may turn out to be much greater than virtually any other car on the market right now, ICE or EV.
In a cold climate with salted roads, rust is what really limits the lifespan of a lot of cars, but with an aluminum body that shouldn't be an issue.
Also, Tesla is the only EV manufacturer on the market that uses NCA cells in their batteries, which should last much longer than any of the other chemistries currently used in mass market EVs (there was a thread somewhere else in the forums that had a great video on this a while back from a researcher who works on li-ion tech). Plus, larger packs should last longer, since each individual cell will be charged and discharged more slowly. Add in the superior Tesla thermal management systems, and it wouldn't surprise me if a new 85kWh pack today is still serviceable 20 years from now (certainly with some capacity loss, but if I had to hazard a guess I'd say you'll still have at least 70-80% capacity left).
So, you don't have to worry about rust, you don't have to worry about battery capacity loss, and I believe I read somewhere that the motor and transmission are rated for something like a million miles. Of course this is all just speculation, and it's possible all sorts of things will start going wrong ten years down the line. But, judging from the high engineering standards Tesla pushes, I could easily see a new Model S lasting 20 years or more. So really it may turn out to effectively be half the cost of a comparable ICE that might only last 10 years.
And when you compound the gasoline savings over 20 years, then the numbers REALLY start to look good...