The expense usually comes in operations. By connecting the water treatment plant to the Internet and making it remotely operable, you can have one guy who sits in an office and is responsible for overseeing the water quality at many different treatment plants. If everything is local, you need one guy on site at each different plant. People are expensive, software is cheap.

Of course, by making it remotely operable, that one guy could be replaced with a guy in Russia who's job is to poison everyone.