We were in Cascais a couple of weeks ago.
We ate at two restaurants, neither of which were expensive and where the food was good.
Pateo do Petisco - http://bit.ly/pateodopetiscocascais
I had an açorda das gambas (traditional alentejo bread stew with prawns which I love) which was to die for! It came in medium sized saucepan and would have done both of us! Cost was about 9.50€
The restaurant is located on the top floor of the old Cascais Market, where as a kid I used to see live poultry and wabbits being sold. The market is now a type of "Space" and there are other restaurants around there that look good as well. If I am correct we came out at about 13,00€ per head including drinks.
One should book as it is clear that it is very popular.
O Apeadeiro - http://bit.ly/oapeadeiro
Found this on our first night. Good Portuguese food.
A bit pricey at the top end of the menu, but I had a good traditional Portuguese meat dish.
Again we both got out for under 15€ per head including drinks.
The food court on the top floor of the Cascais Vila Shopping Centre opposite the station has a range of eateries and on a good day wonderful views over the Tejo Estuary towards Caparica.
There are also two excellent Pastelerias/Cafes in Cascais run by the same group ( http://www.sacolinha.pt/ ) which also do small set dishes of the day (pratos de dia)!
I had soup and a vegetable lasanha type thing for about 5.00€
We go to this one near the Hotel Cidadela:
There is also a branch here: