At my dauhter's school, the children do not bring lunch money. The parent's are billed for their breakfast and lunch once a month. If there are children who can not afford school food, the BOE sends a form to all parents and they can apply for free or reduced lunch. I personally pack my daughter's lunch, b/c it's a bit cheaper than paying for school lunch. To answer your question however, cheap lunch ideas include, mac&cheese (you can buy this for around 50 cents a box or even cheaper in bulk), noodles & canned marinara sauce, ramen noodles (you probably already know how cheap this is), canned soup is a great idea b/c it is also a hot lunch, as well as grilled cheese, and also hot dogs are cheap and a hot lunch. I think it is great that you all help provide lunch, but I would speak to the principal about billing the parent's, if they are concerned about not getting the money they could bill a month in advance. Children bringing lunch money can result in losing it or it being stolen, and no child should have to go without a good nutritious lunch.
I've heard sunflower butter isn't too bad (soy butter is nasty). I guess it's more expensive than peanut butter, but I don't know how much more at the scale you're looking for.
My son's school provides a hot lunch for those who "forget", then bills the parents. If they don't pay at the end of the year the grades are not released.
Our group usually provides peanut butter sandwiches to kids who have run out of lunch money and forgotten to have their accounts replenished. (We buy peanut butter and loaves of bread in very large quantities and the cafeteria staff stores it/makes the sandwiches.) The idea is to give them something so they don't starve, but not something so good that they continue to conveniently forget their lunch money. Because of concerns over peanut allergies, we're looking for a reasonably priced alternative. The best idea we've come up with so far is bowls of cereal. Anyone else have any good ideas?