It's very easy to forget that we use orders of magnitude more water than comes out of our tap. A pound of beef is about 2,000 gallons, so that would be a nontrivial price increase.

Probably shouldn't be raising cattle in areas that would need desalination to produce those 2000 gallons.

Or maybe such water-intensive food production should require consumption of the planet's nearly unlimited seawater. Pass the greater expense to the cattle producers and/or consumers. Leave the planet's relatively limited freshwater sources for direct human consumption, or food production that's more sustainable, or other urgent and necessary activities. The beef lobby may not agree.

And what of areas where just having water coming out of a tap at all would be a huge improvement?

What? Are they drowning cows now??

I'm looking at this and unclear what he's talking about. Essentially, citation needed Elon?

Israel desalination plant profitably offering a fixed price of 1.45 NIS per cubic meter. At current exchange rates that is around 0.40$ per cubic meter (1000L).

https://www.gov.il/en/departments/news/desalination_260520

Musk made the same "absurdly cheap" claim in a recent interview with Bill Maher.

https://www.youtube.com/watch?v=oO8w6XcXJUs [skip to 14:45]

I don't think he gave any substantiation to the claim then either. (He dismissed delivery of clean water to the world as a problem not interesting enough for him to spend time on.)

Just look at this thread for price estimates. $2-5 is pretty reasonable, even assuming a cost breakdown with energy at 10% and capital/infrastructure at 90% of the final cost.