Gaining

White Bread and Butter. 8-15+ Slices all during the Day between meals. All Carbs and Fats. Cheapest effective fattening snack I can think of.

potatossss
all day erryday
fry em, mash em , roast em , stuff em
go crazy on those taters

Like the others are saying: CARBS. Potatoes, bread, rice, and pasta are all very cheap, and chemically they are practically designed to become fat. For best results, eat them with shameless amounts of butter or oil.

For fast food, Jack in the Box tacos are supposed to be near the top in terms of calories per dollar.

Cheap cuts of meat, and indeed cheap food in general, usually contains more fat and sugar since it's more bother than it's worth the food companies to take it out at source (and sugar of course makes cheap nasty food taste good) unless they sell it as a premium-quality 'diet' range.
And cheap cheese is something like 35% fat!