var px = $("#id").css("marginLeft");// px = '400px'
        var num = parseInt(px);

    var dom = document.getElementById("id");
    var marginLeft = getStyle(dom, "marginLeft");// marginLeft = '400px'
    alert(parseInt(marginLeft));

    function getStyle(obj, attr) {
        if (obj.currentStyle) {
            return obj.currentStyle[attr];
        } else {
            return document.defaultView.getComputedStyle(obj, null)[attr];
        }
    }