Sum costs over multiple groups

[Wiznane]

I have a table set up with a repair order, the cost, date, and unit of that order. It is in a one-to-many relationship with a table holding the repair details.
My resulting query returns something like the following

Unit Month Order # Cost Repair description
1 10 100 $100.00 Oil Filter
1 10 100 $100.00 Air Filter
1 10 100 $100.00 Engine
2 11 101 $250.00 Oil Filter
2 11 101 $250.00 Tires
....
and so on.
Now, in my report, it is grouped first by Unit, then Month, then repair order, then repait description. This works and look lovely, but I cannot get it to sum the costs for the unit for that month.
Putting Sum([Costs]) in the unit footer adds up everything (all the duplicate $100.00s for example), so I figured just putting running sums in one of the headers or footers would do it, but nothing so far.

Any suggestions? Maybe redesigning the query somehow?

 

[Wiznane]

Nevermind. Got it with a
=DSum("[Cost]","tblForkliftMaintenance","[Unit]=[Reports]![rptForklift]![Unit]&quot
control in the outermost (Unit) footer.
Still, if there any design thoughts on this, I'd still be glad to hear it. There's probably an easier way to achieve this.