I think your basic point is dead on, but I'm pretty sure ramen is significantly cheaper than potato for the same amount of calories.
The advantage of potatoes is that there is a large number of recipes that consist almost entirely of potatoes (mashed potatoes, hash browns, roast potatoes, boiled potatoes, potato chips, etc.) so there's a lot of variety in a fixed budget.
Rice ultimately is cheap only because of labour costs. It takes tons of water to grow.