It’s still puzzling to me, and has been since my teens, that some present sustainability as “expensive”. There are so many reasons why that doesn’t make sense.
First, the long term view: what’s really expensive is to continue as we do now. The consequences will be hugely costly in terms of loss of ecosystems, human lives, quality of life, clean up, a very late and essential course adjustment, and more. It is far less costly to change now.
Also, if done right, moving in the direction of a sustainable civilization can be profitable in the short run. It can be done so it’s good for business, communities, science, technological innovation, health, quality of life, and more. There is no reason it should be costly even in the short term. There are many very good solutions that are inexpensive and even saves money. And there are many solutions that are very good for business. (Although often “new” businesses, rather than the established ones.)
The latter is perhaps why this myth has been created or maintained. For established businesses, “business as usual” may seem more profitable in the short run. To them, it may seem that they benefit from it, and from maintaining the myth of “expensive sustainability”. Of course, they overlook the huge potential profit in this area, and that doing a course correction now will benefit them in the mid- to long-term. (And perhaps even short term, again if done right.)
Another reason for this myth may be identity. For some, especially politically or culturally conservative, sustainability is associate with a certain group of people, and this is a group they don’t want to be associated with. So instead of exploring the topic and see that there are ways to do sustainability that fits their existing identity, they reject it altogether.