The cockpit door close mandate only applies to aicraft with 20 or more seats (narrow body aircraft -one aisle- and greater). The reasoning behind this is because smaller than 20 seat aircraft can not do as much damage as a larger aircraft. The goal is to prevent future terrorist attacks such as 911.
The mandate also required installing reinforced doors, the deadline for airlines was due on April 9, 2003. The purchase and installation cost of an enhanced cockpit door is about $12,000 and $17,000. The total cost to airlines was between $92.3 million and $120.7 million over a 10-year period, including increased fuel consumption costs resulting from heavier doors.