Like Joyce says, a lot of meals don't need sides as such. All chilli needs is rice, wedges or cornbread, all of which cost pennies, Spag bol doesn't need anything, although if you add a few salad leaves in season, it doesn't add much to the cost. Stews and casseroles are complete meals in one, perhaps a bit of crusty bread but that's all. I make my own and it costs very little. Same with pasta bakes and what not, no need for sides, except, maybe, some in season leaves.
A lot of the meals that I make are one pot complete meals that don't need anything else. Where we do need something (if I make nut roast or similar) we use either frozen or in season veg and potatoes. These don't add much to the price of the meal. Asda has spring greens for 50p a bag at the moment, for instance. I use these as an accompaniment or as the basis for a meal like spiced potatoes with cabbage, beans and greens or the like.
I agree with Joyce about dessert, as well. We don't really have desert in here. The kids might have a yoghurt, a biscuit or a piece of fruit. Usually, I bake something and they will have that. This week I will bake Mars bar cake. It'll cost about pounds but I will cut it into 60 little squares and they will have one each after a meal. Sometimes I make a pile of little fairy cakes, which adds maybe 50p to the cost of a meal when you average it out. When the meal already costs less than 3quid, an extra 50p still keeps it well under a fiver.
Yesterday, for example, I made cauliflower and bacon soup for lunch, which the kids had with bread. Then they had a yoghurt. For dinner I made meatless loaf (using a Granose mix that I got reduced 
) and made tomato bake to accompany it. Tomato bake is just bread cut into cubes and fried in a little olive oil until crispy, mixed with tinned tomatoes, an onion, some basil, garlic, salt, pepper and sugar, poured into a baking dish, topped with a little cheese and baked until hot, bubbly and crispy. Then they had an apple and a biscuit.
Both those meals together cost about 7 pounds, BUT, there was enough soup left over for them to have it for lunch today, along with the left over loaf, heated and sliced, in sandwiches. So that makes 3 meals averaging out at just over 2pounds each, a little over 50p a serving.
Tonight, I will make haricot bean cassoulet with beans, carrots, onions, celery, tomatoes and garlic sausage. The only thing that I will serve with it is some bread for the children to soak up the juice. It'll cost about 3.50 all in. Then the kids will have a bit of the tray bake I am making, which will add about 20p to the cost of the meal. Tomorrow, I will make salmon loaf using a tin of pink salmon and will serve it with wedges, peas and sweetcorn. That'll come to about 3.50 as well.
There are tons of threads on here about cutting down your food bills. The main points are usually things like reducing your meat intake: using cheaper cuts (we use things like pork shoulder, lamb leg/shoulder, mince, whole chicken cooked and spread out over a few meals): using meat as a flavouring, like Chil mentioned, instead of as a main feature: planning meals and planning to have left overs which you can use as the basis for another meal: using seasonal fruit and veg and basically being adaptable and open to cheaper foods,