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You need to make clear what you are trying to accomplish. If you want to increase the insulation of the doors/windows, then you are looking at R value. The R value of single pane glass is ~1, and dual pane glass is ~2, depending on the air gap. Higher R glass not only decreases heat transfer (ambient air temp), but reduces outside noise as well.

Single pane "insulated" glass is designed to raise the E value, cutting the transmission of infra-red and ultra-violet light through the pane. It is constructed like an automobile windshield, two pieces of glass with a clear polyvinyl membrane between that has an E coating. The R value of single pane insulated glass is not much greater than single pane alone. If you want to limit the fading and heat generated by sunlight through the glass, single pane insulated will work.

Know what you are after, then calculate the cost for replacement. You might find it is not economically worth it for the small gain you achieve.