Try hawker and street food stalls for dinner after having discounted lunch at some places (for example: fries with snacks in Holland and Belgium, sausage on a bun in Germany and Czech Republic, etc.). Usually just a few euro should get you something fresh and warm that'll keep you going until the next morning. Or try to visit central and eastern Europe. Lots to see and do and a grand meal in a normal restaurant doesn't cost much. For those 70 euro ccuk paid for 2 hamburgers in Paris I can easily live for a week in Poland or Czech Republic.
Oh, and avoid any places where tourists eat, especially in cities like Rome, Paris and Barcelona. One side street further may literally make a 75% difference on your bill. And the food usually gets much better, too.
Not sure if still applied, but restaurants in Paris used to have 2 menu's: one for eating inside and one for eating outside at their terrace. Guess which one was most expensive...