Well, I don't know if it will work, but we did something semilar "as footer"
In the group expander/header section
put a stack panel and add 3 labels
your first "label" should be width data binding to a property of the grid that "calculate the width of all 8 first columns"
the 2 others labels should width data binding to their column width
the amount should be calculated using some kind of linq based on the "groups items"
Also don't forget that you should include in the first label width calculation the fact that the grid might scroll
and don't forget to avoid the user to rearrange the columns order
Hope that could help to start...