In Los Angeles, the DWP tier 1 retail price for a cubic foot is very, very roughly 5 cents or roughly $1.75 for 1000 liters. And DWP's prices are going up (for a host of reasons).
To compare them, it would have to be the price someone pays to obtain a kilolitre of untreated water at source.
California seems to have some water transfers, and I found a report listing prices from $50-$550/acre-foot, or
~1.25 million L (2012 )
That would give an upper bound of (550/1250) $0.44/kilolitre at source.
From the DWP FAQ:
"The Sewer Service Charge (SSC) is based on metered water use"
Bottled water is so incredibly expensive that it really doesn't matter where the water in it comes from -- the production cost of the water in the bottle itself is insignificant compared to the retail price of that bottle.
Besides, it's not like you can water lawns with seawater. When you're at the point where you're having to rely on desalinated water to drink, you're going to need to use it on your lawn as well (if you want to have a lawn).