The battery is being offered at $3,500 and in his presentation musk says that once the gigafactory is up and running in 2017 that price will go down.
It seems that there would also be some utility incentive for this system as it provides significant load smoothing and peak power demand reduction. One thing about California is that to be on a net-metering system, you have to put in a solar system that is comparable to your current usage. If you use a battery then this system could be on *your* side of the meter so you wouldn't have to worry about net metering, can have as big a solar system as you like, and with proper energy efficiency go pretty much completely off grid.
By the way, Tombond, the battery system is 10 Kwh of supply but that is for the portion of the day when the sun isn't shining. If you use more than 15 Kwh of electricity per day and you heat your home and your water with natural gas then you are running a very inefficient mansion (home).
For example, I heat with wood, and my water heat is electric with a heat pump water heater (uses about 1/2 the electricity) and I still only use about 8.2 kWh per day on average.