If you wanted just potatoes op you bought the wrong thing, you can buy them much cheaper.
cooked chips though involve someone being paid to buy them, take the delivery, storage, peeling them, chipping them, oil, cooking them, salt and vinegar, wrapping, someone to cook, someone to serve, the shop rent and taxes, the utlilites, the salaries for all of them inc any manager.
and once they add all that up, they need to work out how much they need to sell them for, based on average sales, to pay all the bills and make a small profit. Hopefully.