Cupolas are designed to help with air ventilation and add elegance to your gazebo, pavilion or other outdoor structure. Cupolas will often be an optional addition to a gazebo which can be made with the same materials and in the same style as the gazebo itself. Cupolas will generally come in three pieces, roof middle and base which are easily assembled. You will want to select a size of cupola which will not overbear the gazebo but will still be large enough that the cupola gets noticed. To decide on the size of the cupola, consider 1.5 inches to each foot of unbroken roof surface. A gazebo style of cupola will also look great on other outdoor structures such as sheds and greenhouses. Stand-alone cupolas will start at about $150 while adding on a cupola to your existing gazebo will probably cost you about $300.