The footer section is not going to do anything for you. Since the header and detail are printed before the footer is run, whatever testing you do in the footer can only affect following records which will pass through the next iteration of the header section.
cghoga is correct. If you use the criteria you won't need the calculated field in the footer. If the only records that make it into the report have a Unit Cost > 1, you don't need to test for that cost within the report.
Also, =Count([Unit Cost]) would count the number of times a record with a Unit Cost field appeared for a particular PartID. Since all your Detail records have this field, it's the same as counting all the records for that PartID which appear on the report.