The $1 chocolate bar I gobbled up today had around 1000kJ in energy in 46g.
By comparison, $1 of potato (at $2/kg) would be around 2,800kJ in 1kg.
So the potato has lower caloric density but higher nutrient density.
In terms of price for calorie and nutrient, the potato is the winner.
Based on this simple comparison, a person on a budget would be best to spend money on potatoes instead of chocolate.
But the chocolate is probably more appetising, is definitely easier to prepare, and provides immediate nutrition.