Based on how long you keep your cars for (8 years on the previous Tahoe), the gas savings alone could pay for this car in itself. At $500 a month on gas, you're spending $6k a year on gas. If you keep this car for 10 years, that will mean you have saved ~$60k on gas, and add that to the remaining value of your Model S ~$20k (conservative estimate at 20%), that would mean your cost of ownership relative to the Tahoe would have been only $20k or $2k a year over 10 years. Compare this to your gas car, assuming you bought the car for $40k and $60k spent on gas, your total cost of ownership comes up to $100k (same as the sticker of the Tesla), but I doubt the $40k car would still have $20k value on it (probably closer to $10k).
Annualized over 10 years, the Tesla makes much more financial sense. If you get rid of the car in less years, the car becomes less worth it but still very close based on your driving habits.
Very true stuff. The kind of thinking I was wanting to evoke here. I'm trying to put pencil to paper so I can justify to a few special few, and more importantly to myself, that it isn't quite so black & white when it comes to numbers. In other words, its not the same as going out and buying a new $100,000 Mercedes or BMW.
The only drawback to your logic is perhaps with the technology, will the Model S be fairly antiquated in 6-7 years (if not sooner). At that time will the new Tesla Model Q go 750 miles on a 10 min charge? I guess that is just part of taking the leap of faith as an early adopter.