May a query update a source table?
You cannot update a field in the underlying table of a form when the control is bound to a function (a calculation in this case). However, you can run an update query, use VBA to update the field, use VBA to write the calculation to a bound control, or use a calculated field (MS Access 2010 and later versions only).

However, you must ask yourself if you really want to store a calculated result. It will depend on whether qty and expenses change over time and whether you need a fixed result for the current values. If qty and expenses will always be the same, just use a query to show the result of the calculation. For example:

SELECT t1.qty * t2.expenses As Total
FROM table1 t1 
INNER JOIN table2 t2
ON t1.ID = t2.MatchedID