It can cost anywhere from a few hundred dollars to a few thousand dollars, depending on the size and type of window you choose, as well as the complexity of the installation. Generally speaking, the more elaborate and custom your window design is, the more it will cost. If you already have a screened porch and want to add windows, it's likely that you will need to hire a contractor to do the work for you. Contractors typically charge by the hour, so be sure to get quotes from several different contractors before making a decision.