Pure cheapness has got to go to a major agricultural grain product.
Buy a HUGE bag of flour and make bread, or a HUGE bag of rice, or corn meal… shop where resteraunts shop if you want to save those extra pennies. And by huge I mean 50lb sack. Make a yeast starter and keep it in the fridge, it will last years.
Now caloric density is different, all fats are 8 cals/gram, so whatever the cheapest fat you can find would be another cheap caloric source. But things like coconut milk and heavy cream, while great for putting on weight, are no where near as cheap per cal as grains.
(of course, we have no protein… protein ain’t cheap mate).