Flour is clearly the winner vs. peanut butter when comparing price per calories. At Walmart.com it is $2.12 for a 5lb (2.26kg) bag which is about 8250 calories. But yeah, processing that into bread (home energy cost and time cost) is tough to calculate and probably depends on your situation.

Still there are in-between options - pasta for instance is more bang for your buck than peanut butter and is much easier to prep yourself than bread.

It's interesting this conversation evolved into one about the most cost-effective sources of calories. Unintentionally, it serves as a poignant reflection of how things are going in our times. Many people will need to consider cost of calories seriously soon, many already do so.