window.onload=function(){

  ppf=.40;//price per square foot, edit this to suit

  df=document.forms[0];
  df[0].focus();
df[2].onclick=function() {
   df[0].focus();
 }   
df[0].onkeyup=function() {
if(isNaN(df[0].value)) {
   alert('numbers only please');
   df.reset();
   df[0].focus();
   return;
 }
   df[1].value='$'+df[0].value*ppf;
  }
 }