<html>
<head>
<script type="text/javascript">
function calcular(cual) {
var total = 0;
for (var i = 0, combos = document.getElementsByName(cual); i < combos.length; i++) {
total += parseInt(combos[i].value);
}
document.form1.textfield.value = total;
}
</script>
</head>
<body>
<form id="form1" name="form1" method="post" action="">
<p>
<select name="sel" onchange="calcular(this.name);">
<option value='0'>Elija Origen...</option>
<option value='1'>Granada</option>
</select>
<select name="sel" onchange="calcular(this.name);">
<option value='0'>Elija Destino...</option>
<option value='100'>Málaga</option>
<option value='44'>Aeropuerto</option>
</select>
</label>
</p>
<p>
<label>
<input type="text" name="textfield">
</label>
</p>
</form>
</body>
</html>