Flushing a toilet will always be cheap. Desalination costs somewhere around $1 per tonne of water.

You could flush a toilet with salt water, the problem is you’d have to pump it all the way from the sea. That’s the expensive part.

	▲	verzali 3 days ago \| parent [-]
		All of England is pretty close to the sea though. But you'd need to build a separate set of pipes to carry all the salt water.