Table math in OneNote?

You can use the Table Sum Add-In; it adds an extra row along the bottom with the column sums.

Before:

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After:

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Usage:

Simply select the table to be summed and click the button in the Add-In tab of the ribbon strip:

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The Onetastic add-in has a macro feature and there is a macro that will let you apply functions to a column such as sum, average, etc. It only puts the result into the cell, it doesn't update automatically if you change some values in the column.

After you install Onetastic, click on the "Download Macros" button. A dialog opens. Enter "function" in the search box. There is a macro called "Function", select it and you'll see its details on the right. Click "Install" and the macro is installed.

Now put your cursor in the cell where you want (for example) the sum of the values in the column above the cell you chose. Click the "Table" button which now appears in the Onetastic Macros part of the ribbon and select "function" from the list, a dialog opens from which you can select "sum", "average", etc. Now click "OK" and the result is put into the empty cell.