Cómo sumar registros de una tabla al seleccionar un checkbox?[No resuelta]

<div id="container">
<div class="panel panel-primary">
    <div class="panel-heading">VENTAS PENDIENTES DE CONCILIAR!</div>
    <div class="panel-body">
        <div style="overflow: auto; width: 1156px; height: 228px;">
            <div class="table-responsive"> 
                <div id="inf_vents" class='alert alert-info'>No hay ventas registradas!</div>
                <div id="ult_vents"></div>          
            </div>
        </div>
    </div>
</div>

$('#inf_vents').hide();
        $.ajax({
            url: url,
            data: {iduser: idu, idtipo: 4},
            type: 'POST',
            success: function( data ) {
                $("#ult_vents").html(data);
            }
        })

$sql = "(SELECT fac.idsorteo, evs.nombre, date_format(fac.fecha_registro,'%d/%m/%Y') AS fecha, ";
    $sql.= "fac.monto_total, fac.monto_a_pagar, fac.monto_comision, fac.monto_neto, fac.sobregiro ";
    $sql.= "FROM (w_facturacion_users AS fac JOIN w_eventos AS evs ON fac.idevento = evs.id) ";
    $sql.= "INNER JOIN w_users_eventos AS uev ON fac.iduser = uev.iduser AND fac.idevento = ";
    $sql.= "uev.idevento WHERE fac.iduser = $idu AND fac.pagado NOT LIKE 'S' ";
    $sql.= "HAVING fac.monto_total > 0 ) ";
    $sql.= "UNION ALL ";
    $sql.= "(SELECT fau.idsorteo, 'ADELANTO LIQUIDACION' AS Nombre, ";
    $sql.= "date_format(fau.fecha_registro,'%d/%m/%Y') AS fecha, fau.monto_total AS monto_total, ";
    $sql.= "fau.monto_a_pagar AS monto_a_pagar, fau.monto_comision AS monto_comision, ";
    $sql.= "fau.monto_neto AS monto_neto, fau.sobregiro AS sobregiro ";
    $sql.= "FROM w_facturacion_users AS fau WHERE ((fau.tipo_trans = 6) AND ";
    $sql.= "(not((fau.pagado like 'S'))) AND (fau.iduser = $idu))) ";
    $sql.= "ORDER BY idsorteo ASC, fecha";

    $qry = exec_sqli($sql);

    if ($qry->num_rows>0) {
        $salida = "
        <table class='table' table-striped' id='tbl_select'>
            <thead class='btn-primary'>
                <tr>
                    <th>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<input type='checkbox'/></th>
                    <th style='text-align:center;'>SORTEO</th>
                    <th style='text-align:left;  '>DESCRIPCION</th>
                    <th style='text-align:center;'>FECHA</th>
                    <th style='text-align:right; '>VENTAS</th>
                    <th style='text-align:right; '>PREMIOS</th>
                    <th style='text-align:right; '>SOBREGIRO</th>
                    <th style='text-align:right; '>COMISION</th>
                    <th style='text-align:right; '>BALANCE</th>
                </tr>
            </thead>
            <tbody>";
                while($row=$qry->fetch_row())
                {
                    $salida.="
                    <tr>
                        <td><input type='checkbox'/></td>
                        <td align='center'>".$row[0]."</td>
                        <td align='left'>"  .$row[1]."</td>
                        <td align='center'>".$row[2]."</td>
                        <td align='right'>".monto($row[3],0)."</td>
                        <td align='right'>".monto($row[4],0)."</td>
                        <td align='right'>".monto($row[7],0)."</td>
                        <td align='right'>".monto($row[5],0)."</td>
                        <td align='right'>".monto($row[6],0)."</td>
                    </tr>";
                }
                $salida.="
            </tbody>
        </table>";
    } else $salida = "<div class='alert alert-info'>No hay ventas registradas!</div>";

    echo $salida;
    return;

let buys = document.getElementById('tbl_select');
    let cboA = buys.querySelector('thead input[type="checkbox"]');
    let cbob = buys.querySelectorAll('tbody input[type="checkbox"]');
    let mtot = document.getElementById('gc_monto');
    let total = 0;
    alert(total);

    [].forEach.call(cbob, function (cbox) {
        cbox.addEventListener('change', handleRowSelect);
    });

    cboA.addEventListener('change', function () {
        [].forEach.call(cboxes, function (cbox) {
        //cbox.checked = cboA.checked;
        cbox.click();
        });
    });

    function handleRowSelect (e) {
        let row = e.target.parentNode.parentNode;
        // let qty = row.querySelector('td:nth-child(3)').textContent;
        let price = row.querySelector('td:nth-child(9)').textContent;
        // let cost = Number(qty) * Number(price);
        let cost = Number(price);

        if (e.target.checked) {
            total += cost;
        } else {
            total -= cost;
        }

        total = Number(total.toFixed(2));
        mtot.value = total;
        // alert(total);
    }