This is a good idea! Do you propose other methods of aggregation, such as min/max/average? Should relevant ones work on non-numeric fields, e.g. to get the latest posting date?

It seems you suggest the user must choose manually to calculate sums, but I'd suggest that auto-loading sums could be specified via a page property during development, or perhaps even by customisation.

Lastly I would suggest that the auto-sum row, if present, should be 'pinned' at the bottom of the list, rather than having to scroll all the way down, but I don't know if that is easy to implement from UI/SQL points of view.