gistfile1.js

// 'message' should be changed to match the name of your output box
var display = $('message').down('p').update();

new Form.Observer(document.forms[0], 0.3, function(form, value){
  var vals = value.parseQuery(), message = '';
  var amt = parseInt(vals.x, 10) * parseInt(vals.y, 10) * parseInt(vals.material, 10);
  if(amt > 0){
    message = (amt / 100).toFixed(2);
  }
  display.update(message);
});