It's like deciding Microsoft's profit margin based on the cost to physically acquire CD-ROMs.
The primary video cost (programming) is approaching 50% of video revenue, and it's growing at a pace that has literally equaled or exceeded video revenue growth for each annual period in the linked document. So, making an assumption here that's likely untrue, if we consider that the other costs are distributed equally between the two (marketing, tech support, physical support, etc) then the internet service is approaching parity with the video service, and will exceed it in several more years as the higher profit business component.
I suppose if someone else has more time to dig through this document or other insight from being in the industry they could chime in with which has the greater costs once programming is removed from the equation.
The actual electrical cost to move the bits back and forth is nil. It's making sure that half the people in the neighborhood can use Netflix at 7pm two years from tomorrow that requires them to spend money today.
In parts of the country, slower-speed copper, fast-download cable, and a few fiber networks are already built out. The cable distribution giants like Time Warner Cable and Comcast are already making a 97 percent margin on their “almost comically profitable” Internet services, according to Craig Moffet, an analyst at the Wall Street firm Bernstein Research. As Levin points out, “If you are making that kind of margin, it’s hard to improve it.” And most Americans have no choice but to deal with their local cable company.
-- http://www.technologyreview.com/news/510176/when-will-the-re...
If that's accurate, I suspect that they could live without cable as we know it today and still make plenty of money. Working on finding out more numbers to see what the revenue breakdown is between cable and internet, though I wouldn't mind if someone else chimed in.
EDIT: Found this [1] for Comcast. For 2012 and 2013 they've lost video customers, increased internet customers. Had growth in revenue for both categories, but a greater increase from internet (percentage and absolute).
So the way things are heading, cable leaving would cost them a lot. However, they can probably make up for it elsewhere, and, ultimately, should be planning on cable TV going the way of the dodo regardless of the nearterm revenues and profits. It's not a viable longterm strategy, like SMS (while immensely profitable) for cell carriers, a good short term strategy but as content shifts to the generic data channels the specialized channels will fall into disuse.
[1] http://files.shareholder.com/downloads/CMCSA/3182774232x0xS1...
EDIT 2: Check out page 53 of the linked filing.
Video revenue for 2013 was $20.5 billion. Programming costs were $9.1 billion. So the net, ignoring any other costs, right there is $10.4 billion. If the ISP side of the house is actually making 97% margins like the claim in the Technology Review article, then it is making almost as much as the video side of the house. At the current growth rates it'll surpass the video profit, but not revenue, sometime in the next few years. Their NBCUniversal acquisition may reduce their programming costs, I'm not sure of the legalities involved in that (their growth and acquisitions could put them afoul of anti-trust/monopoly laws and regulations).