How Do You Order an Aggregate Function

I'm attempting to calculate (in decending order) the cost of inventory of
spare parts which were used last year. I'm doing this using a report which
groups by the part number; however, when I attempt to then sort by the total
cost of those issues and not by the individual lines which make up the
issues, I get the error message, "Cannot have aggregate function in ORDER BY
clause <clause>. (Error 3097)"

Is there a way around this difficulty?

Thanks to anyone who can offer some advise.