To sort subtotals for groups in Excel, you can follow these steps:

1. Select the data range that you want to sort.
2. Go to the "Data" tab in the ribbon and click on "Sort".
3. In the "Sort" dialog box, select the column that contains the subtotals you want to sort by in the "Sort by" dropdown list.
4. Choose "Values" in the "Sort On" dropdown list.
5. Select "Descending" in the "Order" dropdown list.
6. Click "Add Level" to add another sorting level if needed.
7. Select the column you want to sort by in the "Then by" dropdown list.
8. Choose "Values" in the "Sort On" dropdown list.
9. Select "Descending" in the "Order" dropdown list.
10. Click "OK" to apply the sorting.

This should sort your subtotals by descending order of cost. If you have used the subtotals feature to group your data, the subtotals will remain grouped together and sorted within their respective groups.