In our techno-economic modeling process, we assume that battery electric and fuel cell propulsion costs fit empirically observed learning curves analogous to the history of hundreds of diverse manufactured goods. The typical metric is that a product becomes 15–30% cheaper for each doubling of the cumulative number produced, as shown on these two curves. Many organizations and agencies forecast that the current range of $500–800 per kWh could be as low as $400 by 2020. For more, see Wind and solar photovoltaic capital cost trends.
Kromer, Matthew, and John Heywood. 2007. Electric Powertrains: Opportunities and Challenges in the U.S. Light-Duty Vehicle Fleet. Laboratory for Energy and the Environment. link