What is the value of a 2006 five dollar bill with the 2003 design?

$5. Both designs were printed under the same series date.

To explain, U.S. bills are dated by "series", not by year of issue. In modern practice the series corresponds to the Treasury Secretary and Treasurer in office at the time the bills were first printed. When the new $5 bills were introduced the same officials were in office as before, so the same series year was used.