You can make pasta with flour, water, and salt. Add yeast, and you can make country loaf bread. Add a little sugar, butter, & milk, and you can make white sandwhich bread, or dumplings for soup. These are absurdly easy recipes, almost impossible to mess up. Change the portions, and you have sugar cookies, like you said! Splurge on chocolate chips and you can have chocolate chip cookies. Get some baking soda, and you can make crackers.
Flour’s about 80¢/lb. Salt is $10 for 26 oz, which will last many, many recipes. Yeast is $1.50/oz. For $25, you can make about 25 loaves of bread, and still have a bunch of salt left over.
Flour is the single best, and most versitile, calorie-to-dollar value food.