On a typical day, how much money should I plan on spending on food in Spain.
That can vary greatly depending on your budget and how much you eat. I travel on a pretty tight budget, so I spent about €15/day in Spain - fruit or pastry for breakfast, sandwich or picnic lunch, and then a budget restaurant for dinner. Of course, the sky is the limit when it comes to eating in Madrid, Sevilla, Barcelona, etc. There are tons of expensive trendy restaurants where you can drop €50+, or you can eat at a local cafe for about €8-15 for dinner. Be sure to do a "tapas crawl" at least once! And churros con chocolate for dessert!
We live in southern Spain, which is cheaper than other parts of Spain I think. Breakfast I don't know as we are usually in our house or in an apartment in Seville self catering but for lunch we would spend (for 2) 10 - 12 euros - that would be 2 boccadillos (rolls filled with say ham and cheese), 2 glasses of wine and 2 coffees. At a bar generally but sitting at a table and quite pleasant.
Look out for the 'menu del dia' - menu of the day - which is generally 3 courses for a set price of around 7 euros per person. Usually includes a glass of wine and coffee. You will get 2 largeish entree size dishes and dessert - huge amount of food for lunch - but incredible value.
Dinner - we would spend between 25 - 35 euros for 2 which would generally include a bottle of house wine, 2 main courses and a salad. The variance would usually be the dishes we choose - lamb for example is expensive, pork cheap.
And we certainly don't live in the cheapest area of andalucia.