changes to formla triggered by update to drop down

changes to formla triggered by update to drop down

Hello,
I have a drop down list of shirt sizes and a text field for quantity of shirts.  The size ordered determines price.  I am using a formula field to calculate the total cost of the order (quantitiy of shirts * shirtcost).  It works well the first time (user selects size and inputs quantity).  But if the user changes the size of the shirt without changing the quantity of shirts, the forumla is not updated.  Is there any way to force the formula to check for a new value from the drop down?
 
Sonya