jQuery offset combined with jQuery UI doesn't update on drag

Question

I need to get the offset of the element that is being dragged and I use:

$(function(){
    $(".draggable").draggable({
        stop: function(){
            var offset = $("#boxone").offset();
            alert(offset.left);
        }
    });
});

the element is positioned absolute and has a CSS of:

left:100px; 

that is the value I get on the alert when I drag the element (even multiple times).

What I want is to get it to alert the current offset (if I drag in 200px to the right it should say 300)