jQuery Ajax not sending data to PHP

From the PHP documentation on json_decode :

NULL is returned if the json cannot be decoded or if the encoded data is deeper than the recursion limit.

So most likely there is some bug in your JSON data that is preventing json_decode from parsing it properly, I've run the snippet through jsonlint and it does say it's valid JSON, but it's worth checking a larger sample of data you're sending Inconsistency to the server.

Other than that, is there any reason to call JSON.stringify on the data object before sending to the server? I would try sending the object itself as the data parameter of the AJAX call like this:

        $.ajax($.extend({}, ajaxObj, {
             data: { "addList": addList },
            success: function (rows) {
                $grid.pqGrid("commit", { type: 'add', rows: rows });

            },
            complete: function () {
                $grid.pqGrid("hideLoading");
                $grid.pqGrid("rollback", { type: 'add' });
                $('#consola').text(JSON.stringify(addList));
            }
        }));

and see if that helps:

edit

I should have noticed in the original answer that you don't need to call json_decode on the posted data, jQuery will correctly encode the data as a post parameter for you; it should be accessible in your PHP script as an associative array, try it in your replace the current var_dump statement in PHP and you var_dump($_POST['addList'][0]['orcamento']);should be good to go.