A bit of Google searching and it seems the replacement cost of the 85 kWh battery is $12,000 (link). In the next 5-10 years, you can expect that price to go down as batteries improve. If you're trying to make a rational model of the used price of the car, you can pretend that you're buying two parts: the shell of the car and a $12,000 battery. Both depreciate at different rates. The battery pack's value drops linearly to zero as you put however many miles on it. The shell's value depreciates more like a car (a big loss, off the lot, a bunch in the first few years, and then probably slows down).

So far, Teslas seems to be mostly targeted at the sort of drivers who have dedicated garage space at home as well as a second car, for inter-city driving. Using a Tesla as your sole car, even once they finish rolling out their American "Supercharger" network, or keeping one with an apartment that doesn't give you the charging infrastructure, is just not where they're at right now.