var item_money = $('#money_field_id_here').val();
var quantity = $('#quantity_field_id_here').val();
var total = item_money * quantity;
var result = total.toFixed(2); //change it from $1 to $1.00
$('#total_field_id').val(result); //put the total into the field
I think you make the problem too complicated. try the code i just gave you.
make sure you revise #money_field_id_here... to specific field id you have.
Content will be erased after question is completed.