Calculating Aggregates in grouped rows

I've created a table with an aggregate and also with grouped rows. I've tried a few different ways to add an aggregate to group rows, but none worked. I was wondering what the best way to use the api to do this is.

A link to the debug page is here: http://debug.datatables.net/ikomex

The grouping code is in my drawCallback function:

drawCallback: function (settings) {
            var api = this.api();
            var depth = 1;
            var rows = api.rows({ page: 'current' }).nodes();

            //initialise last group names array
            var lastNames = [];
            if(api.data().length > 0)
                depth = api.data()[0].AssetIdPath.match(/\./g).length;
            for (var i = 0; i < depth; i++) {
                lastNames.push("");
            }
            
            api.column(1, { page: 'current' }).data().each(function (group, i) {
                var parentGroups = group.split("/");
                for (var g = 0; g < depth; g++) {
                    if (lastNames[g] !== parentGroups[g]) {
                        $(rows).eq(i).before(
                        '<tr class="group"><td colspan="7">' + parentGroups[g] + '</td></tr>'
                        );

                        lastNames[g] = parentGroups[g];
                    }
                }

            });
        }

This shows multi level groups based on the string in column 1, split by a forward slash. For example 1A / 1A.1 would show a parent group row for 1A, one underneath it for 1A.1, and any 1A.1 rows beneath that.

What I'd like to do is add a cost in here

 '<tr class="group"><td colspan="7">' + parentGroups[g] + '</td></tr>'

I'm guessing I need to marry up the each loop with something like this (this is my totalling for the footer)

             api
                .column(6)
                .data()
                .reduce(function (a, b) {
                    return parseInt(a) + parseInt(b);
                }, 0);

Except not using .column() and maybe using .filter()

Anyone have any ideas? I've tried a few things, but don't think I grok the api fully yet. Sorry I can't get a live demo up.