A cheap way to double glaze your window is to use shrinkfilm. Cut some strips of 3mm ply about 2cm wide and tack to the frame:
Heat gun the film to remove any bubbles.
You're done. The left window is filmed, the right window is not. You can see a small tear I put in the fold. There is a little translucency added, unlike a thinner or more even film. But you get what you pay for.
I used PVC film, 100µm I think. Thinner would have been better on reflection, to get a smoother finish. It would also be good to have an antistatic coating on them to prevent dust build up. An IR reflective film would improve the performance. My film cost about 20c / m². Best to avoid folded films, though I couldn't find any big enough.
This technique is apparently common in North America, with specially made films.
Wash the window before installation.