If you buy the electric car new and keep it for 10 years, the cost of fuel saved could easily be more than the cost of the battery. If you assume $2/gallon gas, a car that gets 30mpg would consume $6000 in gas in 90k miles. So an electric car compared to a gas subcompact balances out in 90k for the owner. Compared to a car with lower gas mileage, the owner being ahead with electric happens quicker. Newer batteries which can be cheaper and it happens quicker. Math also changes if gas goes up or down, etc. Lots of variables.
That also means if you buy the car used, the first owner get full advantage of that gas savings and the new buyer gets none of it for the miles already driven. So for a used electric car to make sense it has to be less expensive than a similar gas powered car by the amount of fuel savings already used by the original owner.
In other words, it is really hard to tell what a used electric car should be worth. I would not buy one used unless the battery replacement cost was already discounted from similar gas powered cars.
Electric car technology is advancing quickly - both battery life and replacement cost will continue to improve.