The actual cost has to price in the impact of using it.
For example, it's cheaper for UK water companies to pump sewage into rivers and onto beaches:
https://www.bbc.com/news/articles/cz9kz8ydjpno
https://www.bbc.com/news/uk-england-london-67357566
https://www.bbc.com/news/articles/c5yprnd848ko
https://www.theguardian.com/environment/2025/sep/16/sewage-o...
But maybe it's a nice idea to force them to deal with sewage properly so you don't have to live in rivers of shit.