A former customer had been in the donut biz for many years when he bought a Dunkin Donuts franchise. He did a week’s worth of business on the first day that he opened as a DD in the same location. His unit ingredient costs dropped by a third, and covered a good part of his on going franchise fees. He paid big , big, $$ into the ad fund that he resented at first, but he came to appreciate what the ads did for him as he became rich. The fun ended when he went through a nasty divorce with his wife/partner. I have not seen him in years.
Off hand, I can not think of any hot dog franchise that has the marketing clout of a McDs, BK, Subway, Quizno’s or Dunkin Donuts. You need to make a pro forma, with a franchise and without. A pro forma is not always accurate, due to the old garbage in, garbage out thing, but it can give you a place to start. Before investing, I would also spend a little $$$ for a road trip to visit existing franchises. Don’t say who you are, just take a quiet look around. After some observations, you can introduce yourself and ask questions.