Prueba esto, quizas te ayude.
Código HTML:
<html>
<head>
<title>Untitled Document</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<script language="javascript">
function suma()
{
var s = 0;
for (i = 0; i < 3; i++)
if (document.form1.checkbox[i].checked)
s += parseInt(document.getElementById('txtFld'+(i+1)).value);
return s;
}
</script>
</head>
<body>
<form name="form1" method="post" action="">
<p>
<input name="checkbox" type="checkbox" onClick="document.getElementById('txtFld1').value=this.checked?this.value:''" value="1">
<input type="text" id="txtFld1">
</p>
<p>
<input name="checkbox" type="checkbox" onClick="document.getElementById('txtFld2').value=this.checked?this.value:''" value="2">
<input type="text" id="txtFld2">
</p>
<p>
<input name="checkbox" type="checkbox" onClick="document.getElementById('txtFld3').value=this.checked?this.value:''" value="3">
<input type="text" id="txtFld3">
</p>
<p>
<input name="" type="button" onClick="document.getElementById('txtFld4').value=suma()" value="Cotizacion">
</p>
<p>
<input type="text" id="txtFld4">
</p>
</form>
</body>
</html>