在jQuery / Ajax中的dataString中没有得到正确的值

In my ajax script below when I do console.log for duration it returns <select name="duration" id="duration"> instead of the value. I think for this reason my function is not working properly. I am not getting any response back from the PHP page and the loading image keeps on loading.

$(document).ready(function() {
    $("#submit").click(function() {
        var dataString = {
            quantity: $("#quantity").val(),
            duration: $("select[name='duration']").val()
        };
        console.log(duration);
    $.ajax({
            type: "POST",
            dataType : "json",
            url: "rent-process.php",
            data: dataString,
            cache: true,
      beforeSend: function(){
                $("#submit").hide();
                $("#loading-rent").show();
      },
            success: function(json){
                $("#loading-rent").hide();
                $("#submit").show();
                $(".message").html(json.status).fadeIn();
                $('#wallet-av').html('$' + json.wallet);
                $('#ref-av').html(json.referrals);
            }
        });
        return false;
    });
});