General Breakdown

30% Cost of goods (ingredients, take away packaging, etc)
30% Operations (rent, utilities, salary, etc)
30% Sales and Marketing (ads, promotions, discounts)

Remaining 10% is your profit

Yes, why do you think grab takes a 30% cut? That number is not plucked out of thin air. Generally, marketing is not cheap and competition is high. So your ad costs can be expensive too to get the eyeballs of people on facebook/instagram. Then you have to consider if you want to pay to maintain SEO, or engage influencers, etc. Consider that discounts like lunch specials, technically also fall under this category.

Of course 30% is just a general guideline, can be lower/higher... but normally its within that range. If you have other factors that help you gain organic growth and customer loyalty then you may no need to spend so much.

The problem with franchise is that you're very much at the mercy of the brand's marketing, so your ability to generate sales of your own is quite limited. However this also depends on the arrangement with the brand owners, I know some franchises are given more freedom.

Though keep in mind that even if well branded, you'll still need to spend some money and effort in local promoting, can't 100% rely on the brand. Better to go in with a mindset that you need to promote than to rely on external forces, otherwise you almost guarantee mati.