O Golfinho, literally right outside of LxFactory. You'd have to work hard to spend more than 12 euros a person. Simple, local food. Lots of specials (soup, sandwich, fries, beer for like 9-10 euro). Run by an older couple who spoke no English and treated us like we were their niece and nephew. I think both of us got in and out of there for under 20 euro combined.

For a bit higher-end, we liked Cantinho do Sol near the Marques De Pombal circle. I think we spent maybe 20-25 euros a person there, were stuffed to the gills, and nearly drown in our drinks. Employees were lovely Brazilians in the front of house and the owner, a local I think, was the chef.