We have always done spaghetti feeds as fundraisers. It can be super duper easy if you use jarred sauce, and just add italian sausage or ground beef to it. If you have time to cook sauce, that may be cheaper, depending upon how you make it.
One really easy dinner to make ahead is enchilada casserole. You mix enchilada sauce with equal parts of tomato sauce (so children and elderly do not get indigestion, dip the tortillas in the sauce, lay them in a pan in a single layer, much faster than rolling individual enchiladas. Top with beef or chicken, cheese and refried beans. Repeat the layers one more time., top with cheese and bake. They are good with green sauce and chicken also. Serve with salsa and sour cream, if desired. Ok, now I'm hungry!
Just make mexican or white rice, add fruit and viola!!
Another super easy, feeds an army meal, is, for every 6-8 people you want to serve, cook 1 lb of elbow macaroni. Cut up 1 lb of kielbasa sausage, in small bites for every pound to two of noodles. Add desired amount of chopped onion and green pepper, garlic, salt and pepper.
Cook vegies and sausage in a big pot, in olive or corn oil, until sausage is browned and the vegies are tender. Season to taste. Add the noodles, stir and serve. This is surprisingly good and filling! Cheap too, as I made this many a night to fill up groups of teenagers at my house. (The neighborhood hang out).