If you're cereal eaters, you could try making your own - bulk buying nuts, rolled oats, dried fruit etc. will let you make a nice muesli pretty cheaply. Muesli can be good for adding a little interest to other types of breakfasts too, eg. eating fresh fruit and homemade yoghurt with a little muesli scattered over it.
I'd make up a big batch of salad dressing since that keeps for ages in the fridge, and make salad a staple food for the month. There are loads of vegetables that work well in a salad, so that gives a wide range of options for choosing from whatever is in season and cheap. If your family are meat eaters, there are a few good cold meat options that work in salads as well, eg. smoked chicken or fish. (That might be too pricey, but I have lived somewhere where this was actually pretty economical, so it's just a thought.)
It's not the most nutritious meal in the world, but pasta is a good alternative to store-bought convenience foods also - you can either microwave it and throw in some veges (using heated tomatoes to create sauce) or if you can't face that in the heat, make a pasta salad out of it (again, with veges). Cheap and easy!