After my first Camino, in 2018, I spent a week in Porto. Across the street from my AirBNB was a small neighborhood family bar that had a 10 Euro menu del dia. The best part, after getting off the bus from SdC, was the half liter of cold, crisp white wine that accompanied it.
There are several local specialties that don't cost much. Francesinha is a multi-layered pressed sandwich filled with meat, covered with melted cheese and and tomato sauce, and is often accompanied with a fried egg and/or fries. It's enough calories to fuel your first day of walking!
The other local specialty is cachorrinha, a spiced portuguese pressed hot dog with melted cheese and a spicy sauce. It's cut into pieces and is enough to share for some people. It's served with fries and cold beer. Cevejeria Gazela where, I ate mine, was featured in Anthony Bourdain's Parts Unknown segment on Porto.
In 2018, the francesinho cost 8-12 Euros depending where you bought it. The cachorrinho was less.
Buen provecho. Have fun.