OK, so you have family coming for dinner unexpectedly and you don't have much left to spend this month. You don't want to put a lot of expensive stuff on the credit card or take them out to have a nice dinner, but you still want to do something nice for your family.
A lot depends on how broke you really are, what you have on hand, and how much time you have to prepare. And just who is coming. Your elderly grandparents won't necessarily appreciate the same meal that would go over well with families with small children and probably have different dietary considerations.
If you're only a little tight on money: For the family with small kids, if you have a grill and not much time to prepare, consider grilling hot dogs for the kids and grilling chicken or steak or pork chops (whatever is on sale or in your freezer) for the adults. Make a salad to go with it, bake potatoes on the grill, and make a quick cake or cookies for dessert. For your elderly grandparents or even your sister and her husband, consider baked or roasted chicken, a salad, freshly made bread of some kind or pasta, a vegetable side dish or two, and fresh fruit for dessert (if strawberries are in season and cheap, wash, hull and slice them and serve with just a little sugar sprinkled on them).
If you're really tight on money but have more time, consider homemade spaghetti, with a cheap salad made at home, a side dish of vegetables (corn or beans), homemade garlic bread, and a cheap cake. You should be able to make this meal for 4 people for about $7-$8 or less. Homemade spaghetti sauce is easy, by the way. My mother made it with two cans of undiluted tomato soup, 1 tblspn of brown sugar, 1.5 tblspns of chili powder, and some bay leaves and whole cloves (make sure you remove these before serving) simmered for up to an hour with some browned ground meat. And garlic bread is just a loaf of bread sliced in half, buttered and sprinkled with garlic salt before toasting.
Another option, depending on who's coming, is homemade soup and homemade bread. I've discussed making soup before. Homemade bread is also pretty easy, and I'll post a couple of recipes sometime soon. But on a cold day, there's nothing like a bowl of hot soup and warm bread (not to mention the aroma of bread baking). Add some fresh fruit for dessert, and you have a very satisfying meal for four, possibly for less than $6, depending on what you use. This is usually better for adult guests than kids, though. But you should have enough "pantry" foods to whip this out without running to the store, except maybe for the fruit.
If you're mostly having children, consider baked macaroni and cheese, a salad, and a cheap cake. Mac and cheese is cheap, but the baked kind makes it seem a bit more upscale for the adults. And all you need is two cups (pre-cooked measure) of cooked macaroni, 2 cups of cheddar cheese, and 2 cups of milk and 1 egg beaten together. Grease a casserole, layer the mac and the cheese, then pour the milk and egg over it. Bake, covered, at 350 for about 40 minutes.
Oh, and my favorite quick cheap cake recipe: Mix together 1 cup of sugar, 2 tbsp of shortening or melted butter or margarine, 1/2 tsp of salt, 1 cup of milk, 2 cups of unbleached flour, and 1 tsp of baking soda. Grease and flour a pan, and sprinkle cinnamon sugar over the top. Bake until the center is done at about 350, usually about 35 minutes. Makes a nice quick dessert without a lot of heavy icing for about $.60, possibly less. It has no eggs, and you can substitute oil for the butter or margarine if you have a guest with health concerns.