which dishes gain the most money?

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.

I have a different theory. I believe that it is not profit per serve but profit per hour spent preparing that is important. My reasoning using the example above of Gello Mold, just basic.

Using a similar formula:
(total earnings - costs)/hour

Gello Mold:
1200 - 475 = 725
725 / 12 = 60.41 per hour of cooking