Musk went as far as saying that the costs will go as low as under $200 per kilowatt, when asked by a JP Morgan representative. He led people into thinking that the “not-so-distant-future” will be as soon as 2015. That conclusion is derived from Tesla’s plans to start building a third-generation Roadster (after having said they’ll quit making it last year).
A few years ago (not too many, only a couple), the price per kilowatt was revolving around $500 to $600. That’s what it costed General Motors for the battery units they put inside the Chevy Volt.
What Musk said is not far-fetched or unrealistic at all. Of course battery prices will go down, mass production also apply its price-dropping laws in this area, too. Going farther with the idea, I think that 2020 will even see prices for kilowatt going far below $200, maybe $60-$100.
Technology will play a huge role in this process, making the energy storage units cheaper, lighter and more powerful. This is also useful for the grid-regulation industry, where prices matter the most, since the storage solutions are the ones that ultimately affect the price that the end user has to pay for the kilowatt he receives.