About lunch, you can find menu del dia for less of 10€ easily unless you're very downtown. In neighbourhoods (and if you really look, downtown), you can find good menus del dia for 7 or 8 euros. 9 should be even a bit too much, for someone who takes a look around before selecting bar/restaurant.
We spanish use to have dinner at home, unless we re having fun in a weekend, and then have tapas, or that it is a weekday but we make an appointment for some cañas.
There is not such thing as a menu del dia for night purposes except in Chinese restaurants, but there is "platos combinados" which is a very good alternative (a big dish with meat or fish or/and ommelette or burger, plus fried potatoes or small salad, ...) that is usually about 4,5 to 6 euros. If you want beer and a dessert, however, that'll be extra. Beer would be around 1,2 €, dessert is expensive: 2 to 3 €.