How did you come up with that number? Total cost of the battery divided by its capacity?
Since you do not dispose of the battery upon first discharge, the true cost should be amortized across the thousands of cycles it would go through during its lifetime.
Batteries are still far from economical in many situations, but there are many situations where they now make sense. They are also getting cheaper all the time, and as they do, so will the range of applications increase.
