Additional items:
Eggs. Cheap at home expensive at the restaurant.
All drinks. Like water, milk and pop.
Toast, and pbandj sandwiches.

Waffles. Eggs. Hot dogs.

Pancakes.

Egg McMuffin, at our house we use raisin cinnamon English muffins, double Canadian bacon (Peameal bacon) I’ve got a round egg thingy for frying the egg.

Potato skins (appetizer). Bake the potatoes, cut in half, dig out the insides. Put cheddar cheese and bacon bits, bake more to melt the cheese. Add sour cream. Voila.

Sushi! It is waaay cheaper to make it yourself. The rice is inexpensive and as long as you stay away from the really exotic fillings you can enjoy a nice meal at ¼ of the restaurant price.

A cup of coffee is much cheaper too. A cup costs about 10–15 cents if you make it at home.

Very little. Your potato example is probably the most valid. When you factor all the costs, especially for a single person or couple it’s usually cheaper or break even just to go to the restaurant. That falls apart for larger groups though. Seems like potatoes, rice and drinks are where restaurants make all their money.

@Blackwater_Park: I think as far as profits go, desserts are very profitable, as well. They charge 6 to 8 dollars for a slice of cake or pie, or some type of pudding, whereas you can make a cake or pie for a few dollars, and same with pudding. They can get 8 to 12 slices out of a cake or pie, so that’s at least 48 dollars total each.

It also kills me to pay 10 to 14 dollars for a salad when you can make one cheaply at home. Even if it’s a side salad with just basic lettuce, tomatoes, cucumbers and maybe some peppers, that costs about 6 at a restaurant and it’s so cheap to make.

I forgot about salads, they make a small fortune on them. Desserts are hit or miss IMO. There is an Italian place here that makes amazing cakes from scratch and it’s like $8 a slice. I don’t think they’re making much on them considering the time it takes.

A whole roasted chicken is like the protein of 5 entrees, and is really easy.

Lobster

Anything on 99% of breakfast menus, where the hardest skill is flipping an egg.

Improve your cooking skills and every restaurant dish will be cheap and easy!

Soup is another food that can be made really cheaply but will cost you 6 to 8 dollars for a cup or bowl in a restaurant.

@Smashley and a whole chicken is like six bucks.

You can replicate Pizza Hut’s original breaded wings. You can also create their sauce wings by frying them, dipping them in sauce, and baking those wings for about 10 minutes in the oven. They are totally delicious like this.

Pasta.
So my friend loves Olive Garden and since it’s a place where there are options for both of us, we go there together a lot. Lately, I only buy their soup and breadsticks because I can’t easily replicate the breadsticks at home, but their pasta is so damn simple that I refuse to pay for it anymore. You can get a box of pasta for a dollar or less usually in whatever shape you want. If you like veggies, some grocery stores have old vegetables at a reduced price. Even just using half an onion and half a zucchini, for example, wouldn’t cost too much though. Sometimes I add some chickpeas… legumes are a very cheap source of carbs, protein, and iron. As for the sauce, you can do lemon and oil with dried herbs or just a cheap red sauce. It’s so simple. Just remember to salt to pasta water ;-)

My daughter likes meatballs from our local pizza parlor. Three meatballs are $6, and spaghetti and meatballs are $16. I tell her I’ll get the meatballs at the place, and they have sauce, and I’ll come home and make spaghetti. I explain to her that the logic is a whole box of pasta costs $1, and I’ll be goddamned if I’m going to pay $10 for spaghetti from the pizza parlor. The meatballs are worth it, the spaghetti is not.