Batteries are following Moore's law

The installed price of lithium ion batteries is dropping in half every 18 months and they last twice as long ... just like Moore's law for computer chips...
well not quite Moore's law but something very similar.

I've worked for a handful of lithium ion battery-using companies in electric transportation and grid storage where the price is now down to about $500/kWh to purchase. The price has declined like this in the past few years
(not official company info but mine):

You can decide for yourself how how many data points that you need to see where the industry is trending and (much more importantly) if the trend is sustainable. [Edit: got a quote in mid-June of 2013 at $350/kWh. While that's a little bit above that curve, we haven't negotiated yet either.] [Edit2 2013-08-19: I understand that Tesla is buying it's 80kWh packs at $150/kWh or $12,000, which if true is WAY under that curve...so it might not yet be sustainable today but continues the trend until they are sustainable by possibly late next year(?).]

An implication:

An electric sedan gets about 4 miles for each kWh. So 100 kWh of max storage capacity in 2017 would give a 400 mile range with (today's) capability of a 15 minute charge time for about $10,000 (unlike about $30k or more today).
So, keeping in mind that $0.20 per mile of gas is $0.03 per mile of electricity,
in 7-10 years from now there won't be a reason to have a gas car.

(assuming there are enough places to charge it).
See Tesla's Supercharger plans:

Tesla unveils its planned nationwide Supercharger network
[place the usual legal mumbo-jumbo about not representing my employer here]

edit: As of 2016–05–31, we can get battery energy storage systems for around $150/kWh so the trend seems to be holding. The technology around those prices is not always lithium ion however and can be other types, such as flow batteries (which tend to last, effectively, forever).