1) In order to use the method you posted (report->group selection->top: 100->sum of {@cost}->descending) I had to insert a summary on the totalcost field. It is just a detail level field, I created a simple formula to give me the required information and placed it in the details section. When I inserted the sum I was forced to create a group, so all I did was suppress the group header and footer so that the report still looked the same then used the method listed above.
2) totalcost is not a summary, it is a simple formula placed in the details section. I can't insert a group then sort because I am originally sorting by the unit cost and that is what I need to group by, but I don't want actual groups, I need every itemnum to be separate from all others.