The Tesla powerwall is currently $3000[1]. There are installation costs etc on top of that of course though.
Solar is separate - I'm talking about charging the battery from the grid.
Brownouts are expensive, but so is the cost of making sure they don't happen. Peaker plants cost more than $10m/year without even turning them on[2]. There are over 40 in California[3] currently available. Hopefully they don't all cost $10M each to keep available, but it doesn't take many years to surpass that $10B cost...