$(document).ready(function() {
  var thisValue = $("select#pname").val();
  var thisText  = $("select#pname").val();
  thisOpt   = document.createElement('option');
  thisOpt.value = thisValue;
  thisOpt.appendChild(document.createTextNode(thisText)); 
  $("select#pprice").append(thisOpt);
  $("input#psum").val($("select#pprice").val() * $("select#piece").val() );
  $("select#pname").change(function(){ 
	$('select#pprice option').remove();
	var thisValue = this.value;
	var thisText  = this.value;
    thisOpt   = document.createElement('option');
    thisOpt.value = thisValue;
    thisOpt.appendChild(document.createTextNode(thisText)); 
    $("select#pprice").append(thisOpt);
    $("input#psum").val($("select#pprice").val() * $("select#piece").val() );
  });

  $("select#piece").change(function(){ 
    $("input#psum").val($("select#pprice").val() * this.value );
  });

});