I think what you want to do is use the GetSummary function, which allows you to use the value of summary fields in a calculation. So you'd have something like:
(GetSummary(total_total ; breakfield) - GetSummary(cost_total ; breakfield)) / GetSummary(total_total ; breakfield).
The breakfield is the field used for that subsummary part. If it's a Trailing Grand Summary part, use the same field as the first and second part of the GetSummary argument (e.g., GetSummary(total_total ; total_total) - GetSummary(cost_total ; cost_total).
Ann Arbor, MI