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.