<form name="something" action="somfile.xxx" method="post">
<input type="checkbox" name="quote_0" onclick="updateTotal();" /> Bla bla bla
<div id="totalCost"></div>
</form>
<script language="javascript" type="text/javascript">//<![CDATA[
var numQuotes = 1;
var quotePrices = [4.5];
function updateTotal()
{   var total = 0;
   var quote = 'undefined';
   for (var i=0; i<numQuotes; i++)
   {   quote = document.forms['something'].elements['quote_'+i];
       if (typeof quote != 'undefined' && quote.checked)
       {   total += quotePrices[i];
       }
   }
   document.getElementById('totalCost').innerHTML = 'Total cost: '+ total;
}
updateTotal();
//]]>
</script>