Adding function to DataReport for items from two different tables

[KAJTPY]

I am creating the sales report function for a rental program. Items are bought once on the first invoice they were every used on and then rented out multiple times on additional invoices. Right now my data report shows like this.

Invoice #
Item #1 Description Rental Rate
Cost New(if bought on this invoice,
shows nothing if already
in rental inventory)
Item #2 Description Rental Rate
Cost New(if bought on this invoice,
shows nothing if already
in rental inventory)
Etc.
-----------
GrossProfit(Sum of Rental Rates
from Rental History
List Table)
-----------
Total Cost(Sum of new costs for
this invoice from New
Inventory Table)
-----------
Net Profit(Would like this field
to do GrossProfit -
Total Cost. But since
they are from different
tables I cannot merely
insert a ReportFunction.

Any help would be greatly appreciated...