As @maggie1 wrote just adding a second glaze or sheet of perspex without addressing the metal frame is probably a waste of money no matter how much space is between the layers.
For aluminium framed windows doing it properly requires an insulating layer be applied to the metal frame as well as the second glazing, and depending on whether the window is fixed, sliding, or tilting the outcome can vary. Wood framed windows are obviously different, but that issue remains.
Original equipment double glazed windows are a different product to retrofitting, not to be confused.
There are a number of retrofit products on the market, some better than others, but none are ‘cheap’. I got some retrofit quotes 2 years ago and my estimated payback was over 100 years and double that for replacements not retrofits. I have fairly large windows and 4 sliding doors none that are are floor to ceiling and my retrofit quotes were in the order of $2500-4000 per window for perspective. If that quartered (75% reduction) my heating/cooling bill (which is unlikely) the payback would be more than 30 years. They forecast a realistic quarter (25%) reduction in my energy at best. Other than the ‘value’ of making the interior a bit more comfortable (more even temperature and fewer drafts around the windows) why would we do that?
