It's still quite expensive: https://en.wikipedia.org/wiki/Victorian_Desalination_Plant#C...
And that's (currently) depending on cheap Coal generated electricity.
It's expected to produce 150B liters/year. That's $0.00146/L or $0.0055/gal.
Imagine switching to solar doubles the ongoing cost. This is conservative since solar isn't that much more expensive and there are costs other than electricity, but lets go with it. Then the cost goes up to $280M/year and we're talking $0.007/gal.
That's really very cheap.