It takes me about 7 minutes to fry a hamburger patty on my Griddle (to rare!), ignoring the heat-up time and clean-up time. The actual cooking is quite fast. On the other hand, I can end up waiting an hour in line at In-and-Out. So while I agree that it's not an apples-to-apples comparison, the economics articles I've seen that compare bsed on fully loaded costs (to the best that they can) seem to conclude that fast food can be about 10-20% cheaper than grocery.
well, akshually... https://www.fda.gov/food/process-contaminants-food/acrylamid...
however the main concern with fried potatoes is cardiovascular, not cancerogenic.
The story of frying and cardio is still ongoing; I've seen several full reversals in the public health field over the past 30 years. It's really painful being a quantitative physical biologist watching the press around papers that when carefully inspected provide little to no evidence supporting their position.
To your second point: This is where exact apples to apples comparison breaks down. The sane home cook skips deep frying at home and associated hassles unless it's a special occasion. Microwave the potatoes or boil. Fast, minimal cleanup, and now it isn't junk food either.