Cyclingnews just produced a nice article on the relative savings of various bits of aero equipment. Not enough data to be statistically significant but it gives you a rough idea of how much each watt you save costs.....
http://www.cyclingnews.com/tech.php?id=tech/2008/features/jeffs_need_for_speed08
http://www.cyclingnews.com/tech.php?id=tech/2008/features/jeffs_need_for_speed08