Scroll down this webpage for a chart on time saved per device.
http://damonrinard.com/aero/aerodynamics.htm
Whether its worth it or not depends on things like length of race, whether you trying to qualify for something, how windy it is that day. Barleyrocket had it right. A good coach can help you shave a lot more time off than expensive bike components (I know, they are so much fun).