It's absurd that we're so dedicated to the god of Capitalism that we've had to invent a special term ("negative externalities") to describe the basic premise of ethics - "being selfish is bad".

The "economic value" of billboards shouldn't even be a question. It's self-evident that billboards benefit the owner of the billboard at the expense of everyone else. Nobody looks at an empty patch of sky and wishes there were an advertisement there instead.