My easiest and cheapest is probably doing spaghetti and meat sauce (brown half a pound of ground beef with some onions and garlic, add canned sauce and boiled pasta) but I'm pretty sure everyone knows how to make that one, so I'll get us started with homemade mac & cheese!
Boil elbow pasta (I do about a cup for two people, I eyeball it really)
Melt butter in an oven safe pan, add a few tablespoons of flour to make a roux
Add about half a cup of sour cream to the roux
Add half a cup of shredded cheese (I like Parmesan or Swiss, but you can play around! If you're feeling adventurous you can add a sprinkle of blue cheese or a few slices of brie to this also!)
Melt all of that together, stir in the elbow pasta, and season to taste (salt, pepper, garlic powder, whatever sounds good to you!)
Here's where you get to customize it: I like to add frozen peas, corn, or shrimp. I've also put in cherry tomatoes. Again, whatever sounds good to you!
Mix bread crumbs with a little melted butter, and spread over top.
Bake at 350 for half an hour - delicious, super easy, and can be really cheap!