A 2012 blog article from Stop The Cap quotes the Wall Street Journal (without a link) as saying:
Cable executives and analysts say that about 90% of the money cable operators charge for broadband goes straight to gross profits, since there are minimal operational costs for providing Internet service.
It appears to be referring to 2011 figures.
A 2015 Huffington Post opinion piece claimed (with calculations) that Time-Warner was earning 97% profit margin on their high-speed internet service in 2013:
In our Petition for Investigation of Time Warner Cable (TWC) and Comcast, we point out that TWC’s High-Speed Internet service has a 97 percent profit margin and a number of people asked how that statistic was derived. Simple. Time Warner Cable provides the information, (with some caveats).
This references SEC filings but I don't know anything about these types of records, so I don't feel like I can take it at face value.
Are broadband providers in the USA earning 90%+ profit margins on their supply of broadband internet services? (Or, at least, were they around 2011-2013?)