I will be very honest with you, and tell you that I don't know if the Portuguese adapted the idea from Spain or not, what I really know it's that the restaurant boom started in the 80's/90's. There were some restaurants, but not many has the numbers from today. We are the country in Europe with more restaurants/snack-bars per meter square. It could be great, if the VAT wasn't so high, and if there were enough costumers for everybody. Anyway, that’s not what this post it's about.
Usually, depending from the restaurant and the place, the day menu (Menu do Dia) can go from 4,50€ up to 10€. Cheap doesn't mean less quality or badly served, has expensive doesn't mean good food. I usually say, if you want to eat good abroad, try to see where all the locals go to eat. If it's a small place, then a small place it is! Pilgrim menu in Portugal, it's more common in the North, after Porto.
About Portuguese food, @
gamblerqza, everybody talks about cod fish. I don't like it, but hey, give it a try. You can also try grilled fish (anything, from sword fish to salmon), grilled meat (on charcoal, like a barbecue), Cozido à Portuguesa (Portuguese Boiled, with chouriço, multiple types of meat, like pork and cow, potatoes, many vegetables, rice), game dish like Coelho à Caçador, I don't know, you have a lot of things!
The best thing for you, it's to come here and taste it!
Best Regards
Diogo