Parece que ya e logrado hacerlo funcionar yo solo
me quedo algo así:
Código HTML:
<html>
<head>
<title>Calculador de productos</title>
<script>
function cambia1(form1){
if(document.form1.dis[0].selected == true){
document.form1.cos_dis.value="0";
}
if(document.form1.dis[1].selected == true){
document.form1.cos_dis.value="5";
}
if(document.form1.dis[2].selected == true){
document.form1.cos_dis.value="10";
}
}
function cambia2(form1){
if(document.form1.pap[0].selected == true){
document.form1.cos_pap.value="0";
}
if(document.form1.pap[1].selected == true){
document.form1.cos_pap.value="2";
}
if(document.form1.pap[2].selected == true){
document.form1.cos_pap.value="7";
}
if(document.form1.pap[3].selected == true){
document.form1.cos_pap.value="2";
}
}
</script>
</head>
<body><form id="form1" name="form1" method="post" action="">
<table width="414" border="2">
<tr>
<th width="90" rowspan="2" scope="col"> </th>
<th width="82" scope="col">Discos</th>
<th width="86" scope="col">Papeleria</th>
</tr>
<tr>
<td><center>
<select name="dis" id="dis" onchange="cambia1(), calcula('+')">
<option value="0">Ninguno</option>
<option value="5">CD</option>
<option value="10">DVD</option>
</select></center></td>
<td><center>
<select name="pap" id="pap"onchange="cambia2(), calcula('+')">
<option value="0">Ninguno</option>
<option value="2">Lapiz</option>
<option value="7">Pluma</option>
<option value="2">Borrador</option>
</select>
</center></td>
<td width="126"><div align="center"><strong>Total:</strong></div></td>
</tr>
<tr>
<td height="26"><strong>Costo:</strong></td>
<td>
$
<input name="cos_dis" type="text" id="cos_dis" value="0" size="3" readonly/>
</td>
<td>
$
<input name="cos_pap" type="text" id="cos_pap" value="0" size="3" readonly contenteditable="false"/>
</td>
<td>
$
<input name="total_cos" type="text" id="total_cos" value="0" size="5" readonly/>
</td>
</tr>
</table></form>
<script language="javascript">
function calcula(operacion){
var dis = document.form1.cos_dis.value;
var pap = document.form1.cos_pap.value;
var result = eval(dis + operacion + pap);
document.form1.total_cos.value = result ;
}
</script>
</body>
</html> Gracias de cualquier forma a cualquiera que me haya querido ayudar
