I wrote an article a month or two ago outlining foods with a high protein:dollar ratio. This is the calorie:dollar article.
With 1,100 calories/USD, peanut butter is not fucking around. It's cheap. It's high in protein. It requires no preparation. On top of all of that, it's delicious. If you have a peanut allergy, I'm sorry. You're missing out on this glorious substance.
The nectar of weightlifting gods is whole milk. It's 1,000 calories/USD and, like peanut butter, it requires no preparation, it's easy to consume, and it's high in protein. Make space in your refrigerator for milk, because you will need it.
A word of warning if you're trying to gain weight, don't try to drink a ton of milk the first day. You have to work up to it just like with weight on the bar. Start with daily amount you can handle and add a cup every couple of days until you get to where you need to be. Chances are if you get sick from too much milk, you're not lactose intolerant, you're just not adapted to that quantity of lactose.
If you are diagnosed as lactose intolerant, lactase milk will work just fine, but it's a little more expensive.
Burger King Fries
No, I'm not joking. Fries yield 360 calories/USD and are available in almost any city. Don't crutch on this too much, because it's low in protein and high in sodium. Buying potatoes and bananas are comparably cheap and provide similar calories.
Eggland's Best eggs have 300 calories/USD and can be prepared in a variety of ways. I've been eating roughly two dozen eggs for the past six year (plus or minus a dozen), and I'm still not bored of them. They also have tons of protein (31g/USD).
There was a craze in the early 2000s where everyone lost their shit and thought eggs increased your cholesterol. This is not so. First of all, dietary cholesterol does not correlate well with blood cholesterol (the one that matters). Second of all, several of these studies were a lash back from the 1990s "eggs are the greatest thing ever" craze; people started eating lots of egg salad which has mayonnaise in it, so the correlate was actually caused by the mayonnaise.
Ground Beef (90/10)
Ground beef (90/10) has 270 calories/USD and like eggs, can be prepared in a variety of ways which can keep things interesting. It's also high in protein and, if you know how to cook, can be delicious.
If you know anyone that is trying to gain weight (I'm looking at you 16 year old who weighs 130lb at a height of 5'9"), show them the wonders of peanut butter, milk, and potatoes, and they too will gain weight like the giants that have come before them.