So, breakfast in the US tends to be cold packaged cereal and milk, at best, and often is a poptart and can of soda. Honestly, this is a lousy way to start a day, and what's more, it's expensive. Regular oatmeal is a lot cheaper per serving, can be cooked in the microwave, and is generally better for you. Or make granola out of it if you have to have it cold with milk.
But I'm a believer in a high protein breakfast. I read a study many years ago that a breakfast that supplied 22 grams of protein with a little fat and some sort of fruit or vegetable gave a person the most stable blood sugar and energy level all morning, and I try to aim for that. A cheap way to do this is a couple of boiled eggs (about 12 grams of protein), a slice of whole wheat toast (brands vary, mine has about 4 grams of protein per slice), and a glass of skim milk (8 grams) for 24 grams of protein. This is relatively low cal (less than 300 calories, depending on the brand of bread) and, for me, costs around $.65. It's better if you have some fruit with it, but I tend to have a little fruit mid-morning instead. Yogurt and fruit with whole wheat toast is a good breakfast, though it tends to be more expensive.
If you aren't picky about "breakfast" foods, I get low fat turkey hot dogs cheaply, cook up a package all at once, and freeze them in pairs. Two of those on slices of ordinary whole wheat bread with maybe a little tomato or salsa make a good, cheap high protein breakfast. Peanut butter toast and a glass of milk is another option (as long as the toast is whole wheat) and probably one of the cheapest.
If you're desperate, and getting enough calories is the real concern, get day old whole wheat bread (or learn to bake it and buy the flour in bulk) and eat it with at least half a glass of whole milk to make sure you have a complete protein. Or bake a couple of potatoes and have them with a little milk. Or fry the potatoes and break a single egg into the pan to hold them together. French toast is nothing more than slices of bread dipped in an egg beaten with a little milk. Keep in mind that the boxed cereals are VERY expensive per serving.