ok es algo asi:
if ($calcular){
echo "<font class='TitleSmall'>Banners Adicionados:</font><br><b>Su Resultado</b><br><br>";
?>
<form method="post" action="adicionar_banner.php">
<table border="0" cellpadding="2" cellspacing="2" width="100%">
<tr>
<td bgcolor="#A2B7BB" width='10%' align=center>Meses</td>
<td bgcolor="#A2B7BB" width='40%'>Banner Tamaño</td>
<td bgcolor="#A2B7BB" width='10%'>Pagina</td>
<td bgcolor="#A2B7BB" width='10%'>Costo Bs.</td>
</tr>
<?
$i=-1;
$rme4 = mysql_query("SELECT * from $ban_tbl WHERE id_usuario='$valid_user' and sta_pago = 0")
or die(mysql_error());
while ($row = mysql_fetch_array($rme4)){
$siteid = $row["id_banner"];
$bannerancho = $row["ancho"];
$banneralto = $row["alto"];
$bannerpagina = $row["ban_pagina"];
$bannermeses = $row["meses"];
$bannercostos = $row["costos"];
$i++;
$mes2 = $mes[$i];
$costo2 = $costoa[$i];
$costopor = ($mes2 * $costo2);
$costopor = number_format($costopor, 2, ',', '.');
if ($bannerpagina == 1){
$sitepagina = "Buscar";
}
if ($bannerpagina == 2){
$sitepagina = "Categoría";
}
if ($bannerpagina == 3){
$sitepagina = "Galería";
}
print "
<tr>
<td bgcolor='#cccccc' width='5%' align=center><select name=mes[]><option ";
If ($mes2 == 1) {
print "selected ";
}
print "value='1'>1</option><option ";
If ($mes2 == 2) {
print "selected ";
}
print "value='2'>2</option><option ";
If ($mes2 == 3) {
print "selected ";
}
print "value='3'>3</option><option ";
If ($mes2 == 4) {
print "selected ";
}
print "value='4'>4</option><option ";
If ($mes2 == 5) {
print "selected ";
}
print "value='5'>5</option><option ";
If ($mes2 == 6) {
print "selected ";
}
print "value='6'>6</option><option ";
If ($mes2 == 12) {
print "selected ";
}
print "value='12'>12</option></select></td>
<td bgcolor='#cccccc' width='35%'>Ancho = $bannerancho x Alto = $banneralto</td>
<td bgcolor='#cccccc' width='10%'>$sitepagina</td>
<td bgcolor='#cccccc' width='20%' align=right>$costopor<input type='hidden' name='ida[]' value='$siteid'><input type='hidden' name='costoa[]' value='$bannercostos'></td>
</tr>";
}
print "
<tr>
<td bgcolor='#ffffff' width='5%' align=center></td>
<td bgcolor='#ffffff' width='35%'></td>
<td bgcolor='#ffffff' width='10%'>Total...</td>
<td bgcolor='#cccccc' width='20%' align=right>$totalmes</td>
</tr>";
print "<tr><td colspan=4><input type='submit' name='calcular' class='txt' value='Calcular'> <input type='submit' name='actualizar' class='txt' value='Actualizar' ></td></tr></table></form>";
}
lo que quiero es que cada vez que cada vez que alguien seleccione un mes y desea calcular su costo me da el resultado para ese item, y al mismo tiempo me totaliza.