codice:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<title>Untitled Document</title>
<script type="text/javascript">
function calcola(mod,qt){
	var m = parseInt(mod.options[mod.selectedIndex].value.split("|")[1])
	var q = parseInt(qt.options[qt.selectedIndex].value)
	var somma = (m*q)
	document.getElementById('prezzo').innerHTML=somma;
	}
</script>

</head>

<body>
<form action="" method="get" name="modulo">
<select name="modello" id="modello" onchange="if(this.options.selectedIndex!=0 && document.getElementById('quantita').options.selectedIndex!=0){calcola(this, document.getElementById('quantita'))}">             
<option value="seleziona">Modello</option>             
<option value="Senza lembi|50">Senza lembi</option>             
<option value="Un lembo|100">Un Lembo</option>             
<option value="Due lembi incollati|200">Due lembi incollati</option>           
</select>
<select name="quantita" id="quantita" onchange="if(this.options.selectedIndex!=0){calcola(document.getElementById('modello'), this)}else{alert('seleziona una quantita')}">             
<option value="seleziona">Quantità</option>             
<option value="100">100</option>            
<option value="200">200</option>             
<option value="300">300</option>             
<option value="500">500</option>             
<option value="1000">1.000</option>           
</select>

prezzo <div id="prezzo">0</div>
</form>
</body>
</html>
se nome modello non ti serve togli la barra (|) ed elimini .split("|")[1] nella funzione