I'd spotted that too.... but with even the correction it still doesn't play nicely...
function calculate()
{
  var rval = document.getElementById("InsertRecordTool_Retail_Value").value;
  var rrate = document.getElementById("InsertRecordTool_Sug_Rental_Rate").value;
  var drate = (rval * rrate);
  document.getElementById("InsertRecordTool_Daily_Rate").value=(drate);
}
/* On submitting the webform, the function calculate is executed */
document.getElementById("caspioform").onsubmit=calculate;