For you scenario it seems that you may use a pivot grid
which will automatically calculate the totals for you. Is there any reason which prevents you from using this control?
As for the insert issue it would be hard to provide a precise solution for it without having a better understanding of the exact implementation. That said can you share with us the markup and code-behind of the page?
One other thing since you mentioned that the data is held on the server but the calculations are done on the client you may try modifying the code so that the grid is bound programmatically as demonstrated here
. The idea is to use a web method/web service which to return the entire data on the client. This will allow you to populate the grid and do some calculations according to your preferences.
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