The food section of the Mercado das Tulhas (also known as Casa das Tulhas) has several small food stalls serving regional snacks and lunch plates at minimal prices. The quality varies but the best stalls do excellent bolinho de arroz de cuxá (fried rice cakes with the cuxá sauce), pão de batata (potato bread), and the various fried snacks that form the street food vocabulary of Maranhão.
This is the cheapest and most authentic way to eat in the historic centre. Come at lunchtime when everything is fresh, point at what looks good, and sit on one of the plastic chairs at the communal tables. A full lunch for two with drinks rarely comes to more than R$30.