In Brussels, you will find supermarkets all around. In order to give you a rough idea of pricing, you should count +/- 45 EUROS/ week per person.
There are plenty of restaurants in Brussels offering all kinds of food. You should count between 10 and 15 EUROS for a main course in a standard restaurant and about 2 EUROS for a soft drink or a beer. Desserts are about 5 EUROS.
A sandwich in a snack is about 3 EUROS and 1.5 EURO for a soft drink.