AFAIK it is possible.
In the Parts List Style Editor, create a new parts list style for cost. Choose the columns that you want in the Parts List, make sure that the Estimated Cost (or whatever custom iProp you are using) is a column. Then in the Default Columns Settings, double-click on the Estimated Cost cell under the Property column. This should open the Format Column dialog. Go to the Substitution column and Enable Value Substitution. Then under "When exists, use value of" select the Estimated Cost iProp and "When rows are merged, value used is" Sum of Values. Save the Parts List style and close the dialog.
Create a parts list and apply the Cost style formatting. What you should see in the Estimated cost column is the estimated cost for one item multiplied by the quantity of the item.
All that is left is to combine items to get a total cost. Open the Styles Editor and select your Cost style for editing. Click on the Grouping button in the Default Columns Settings box. You now want to group items based on your needs. For example, we have a SUB-ASSEMBLY iprop defined for all parts. This can be either MILL, CNC, or HARDWARE. If I choose the first key in the Grouping settings to be SUB-ASSEMBLY, then the parts list will combine all MILL items, all CNC items and all HARDWARE items and give a summed cost for each of the three catagories. If you want a total cost for the project, then use an iProp which all parts have in common to group by. The grouping column has to appear in the Parts List however. So if you want to group by the Project iProp for example, then all the parts need to have the same value for Project and Project needs to be a column in the Parts List.
hope this helps
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