4 ms·Flushing a toilet will always be cheap. Desalination costs somewhere around $1 per tonne of water.by HPsquared 1y agoFlushing a toilet will always be cheap. Desalination costs somewhere around $1 per tonne of water.mr_toad 1y agoYou 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 1y agoAll 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.