I think the best way to find out which of the dishes gain the most money is to calculate how many profit/costumer you'll gain.
You have to subtract the costs from the total earnings and divide it with the servings. Example:
Mystery meat costs $8 to prepare it and you earn $12 , so the profit is 4$. Then you divide it with the number of servings, so you have to calculate $4/12 servings. Now you know, that the profit you gain for every costumer is $0,333.
formular:
('total earnings'-'costs') / 'servings'
Important is that there is always eating to serve, otherwise you can't get money.
My cafe is level 11 now, and I always cook "FANCY GELLO MOLD", because at the moment it has the most profit per costumer ($1,8625/costumer).
Additional I would recommend to cook at the night, because your zombies will not consume as much energy.
