This is a proposal for a non-standard extension to grid tables that would allow to add a footer. The approach would be to use a separator line containing ~ instead of - to mark the beginning of the table foot:

+------+-------+
| Item | Price |
+======+=======+
| Eggs | 5£    |
+------+-------+
| Spam | 3£    |
+~~~~~~+~~~~~~~+
| Sum  | 8£    |
+------+-------+