Main:
Burgers and hot dogs are always a good idea for feeding a lot of people, so are meatball subs with shredded cheese. Lasagna or homemade mac n cheese are also yummy and quick ways to feed a bunch of people on a budget. Make your own megga subs (like the 6ft subs from subway, but cheaper if you make them at home) with wide baguettes, slice into 3-4 inch pieces and have condiments for people to customize their subs
Appetizer:
Taco dip (simple guacamole aka avocado salt and lime, frito bean dip, 1 cup sour cream 1 cup mayo and 1/2 packet of original taco seasoning, shredded cheese, diced tomatoes, shredded lettuce and sliced back olives) I make this for every party with my friends and its always gone in less than an hour...make 2 for that many people
Onion Dip (1 cup onions, 1 cup mayo, 1 cup sour cream, 1/2 teaspoon pepper, 1/2 teaspoon salt...caramelize onions and mix the rest together, let sit for at least 4 hours for flavors to blend.) Very inexpensive dip that tastes way better than the overly creamy store bought stuff
Potato Skins (clean 5lb bag of potatoes, coat in olive oil, sprinkle with salt, bake for 45 min at 400 or until they are soft...let cool for a little bit, cut in 1/2, scoop out a shallow bowl in the middle, fill with shredded cheese and crumbled bacon, put back in over at 350 for 15 min...use potato middles to make mashed potatoes as a pre-party snack!) really good served with sour cream with a bit of garlic powder in it
Veggie Tray with Dip (baby carrots, cucumbers, red peppers, etc)
If you plan things out in advance you should easily be able to feed all 50 people for about $100 if you make the food at home instead of ordering it.