Whether it be glazed doors, fixed pane windows or rooflights, specifying standard sizes of glazing and in standard colours is the most cost effective. Another way to consider reducing costs is to search around for a second hand or unused items. Obviously the options will be limited and you'll need to be sure that sizes available will work with your design, but you may just come across a bargain. Plus reusing something that is destined for a skip is better for the environment!