You just need to know where to look. Eat in bars and coffee shops. Look for vegetarian or whole foods restaurants. Check out the restaurants in the Leidseplein district. The selection includes Greek, Italian, Mexican, Japanese and Indonesian food, as well as more traditional Dutch fare.
Prices are competitive.
Eat frites, or French fries. They're a cheap snack typically served in a paper cone with mayonnaise rather than ketchup. Learn to enjoy herrings - pickled or otherwise. Look for vending machines, which sometimes sell entire fast-food meals. Consider restricting yourself to one restaurant meal per day and shopping in markets and grocery stores for the other two meals, plus snacks.