Changed form field triggers recalculation of other field?

I havea  form with a few fields that do simple calculation, e.g.:

var v1 = +getField( "Amount_1").value;

var v2 = +getField( "Price_1").value;

if (v1 == 0) { event.target.display = display.hidden; } else { event.value = v1 * v2; event.target.display = display.visible; }

But when this field is calculated becasue one of the input fields changes, I als o want to trigger recalculation of another one. This doesn't automatically happen, so I have to delete the field's content when filling in the form and then it gets recalculated. How do I trigger the recalculation of a field?