What do you think about Double Glazing with plastic sheets?
My house is 102 years old and is on the national registry. My windows are all original. They are single pane and each window has two frames. In those days the cost of heating was nothing to consider so the gaps are quite large all around. I added a box casing that allows me to install a layer of plastic film. I buy the stuff that is "shrink-fit" and is practically invisible. I install all of them in an easy afternoon sometime in November and throw the stuff out in the spring. The cost is nothing compared to how well they work.