Convert the column into a Number type and under Summary select Total. The section heading for that column will update to show you the totals of any sub-section values. Note, it will only show the sum the total of that section and not provide you with a total for all sections. Therefore, if you have a series of sections in your outline, you will need to create a "master" top-level section for all of them and convert them to subsections first.
To see how this works, view the films I've created demonstrating this here
(~2MB) and here
(~3MB). The first shows the basics, the second how to use sections and sub-sections to obtain a grand total for the whole outline, by having a "master" for all the subsections.