This article gives specific steps to add a grand total to the Inventory Cost Report, following the instructions in the Custom Report FAQ linked below.

1.  Expand Reports > All Reports > Inventory Reports, then select Inventory Cost Report and click Design Report

2. From the Insert tab's toolbar, drag a new Footer down under FooterBand1

3. In the Dictionary tab on the left, right click Variables and select New Variable, then set the following properties:
Name: Grand_Total_Cost

Alias: Grand_Total_Cost

Init By: Expression

Click the ... button at the end of the Expression field. Remove the existing text string if there is one, and replace it with this, then click OK:
string.Format(FormatMoneyDecimal,Sum(DataSpItemSummary,SpItemSummary.item_quantity * SpItemSummary.average_cost))

Check the box for Read Only, then click OK

4. Drag Grand_Total_Cost from Variables on the Dictionary tab and drop it into FooterBand2, then position it where you would like it to appear on the report.

When you're done editing, click File, Save. Switch back to the Reports list in the other tab, apply filters if desired to show a single Item or Site/Location, and click Run Report. On the last page, there is a grand total of all the Costs that appear in the report.