Say you have a text box with ControlSource of:
=[Quantity] * [PriceEach]
In the Report Footer section, use:
=Sum([Quantity] * [PriceEach])
Alternatively, create a calculated field in the query that feeds the report,
by typing an expression like this into the Field row in query design:
Amount: [Quantity] * [PriceEach]
This gives you a field you can then sum in the report.