I was talking with a buddy over lunch about the Model S, residual value and the battery. An ICE option's value at 100K miles and 8 years in service is determined mostly by the condition of the car. Were the scheduled maintenance items done on time (oil/other fluid changes)? Did the owner treat the car well? Is the interior/exterior in good condition? If the engine runs well then it would be reasonable to assume you can get another 50K out of it. In short, you can look at it and form a reasonable opinion based on condition and previous experience.
Barring any knock down drag out quality issues, the battery will be the 800lb gorilla when it comes time to sell the MS. So, what will happen with these packs?
Let's say Tesla sells 10k cars per year for the next four years. That's 40k cars and thus 40k batteries to contend with. There is a lot of financial incentive to have a well thought out cost effective service solution. That volume doubles at the end of the 85kw-hr battery's warranty period; an even bigger incentive.
Extracting from experience with memory (another fast moving expensive commodity), I would be willing to guess that Panasonic will increase capacity instead of decreasing cost. Looking at SD cards and USB drives, the trend seems to be to backfill with more capacity and maintain a minimum price point. There comes a price point where there is simply not enough money in building the singular item to build it. Call it a price floor for that item.
The cells used in the MS probably play by that same limitation. When you need battery service years down the road the cells used in your battery will probably not exist. Even if they did, there is a relative age difference between the replacement cells and those still in service in your pack. The capacity difference would be problematic. The solution will likely be to replace all the cells in the pack along with either re-flashing the firmware or simply upgrading the battery management boards to current production to match the new cells.
Circling back to the memory analogy, I suspect getting major battery work in the future will cost just as much as it does today. Capacity will go up but the car's internal charger will still only supply so much current thus charge times will go up proportionally with the increased range.
Ultimately this should put the MS' valuation squarely in the camp of aircraft evaluation. Airframes are reasonably stable once the initial new car smell depreciation is done. Most of the valuation process involves maintenance on the engines and where they are in their overhaul cycle. The same might be said for the battery. The real question for me is how the secondary market evolves when the two and three year old cars start to appear in numbers.
The above came from an hour long conversation with a buddy. I'd love to hear differing opinions.