Should I insulate?

[montyjohn]

I wouldn't know where to start assigning value to those other items.
When you say weather protection, do you mean to the window itself? i.e. wood rot form failing paint?

If there, there is a higher ongoing cost of maintaining older windows. Redoing the putty, linsead paint etc. So a value could be assigned to that (not sure what it should be however).

What I think is sad is the number of people who get new double glazing because they think they should.
I know somebody who is replacing their 30 year old double glazing with new double glazing because they want the best thermal protection.
This is not a period property and has plastic windows, so no issues, but their return on investment is going to be crazy small. Far worse than the £8.70 figure, and without many of the other benefits you've listed.

It would be better if people knew what they were getting.

 

[Cubist]

Notionally windows are not absolutely essential, a wall does much the same job in large measure. Their primary function is of course to allow daylight to enter the rooms concerned, but besides this they also serve to block rain, snow and wind entry - which would likely do severe damage to the fabric of the property and your possessions if given free reign - while affording some resistance to heat loss and also providing some small security enhancements. Ascribing a financial value, as a share of the purchase and installation costs, to each of these features/functions is, of course, a matter of subjective opinion but they should/must be considered when assessing the benefits.

Is there a higher on-going cost? Theres little to no difference between single or double glazing beyond routine painting and ensuring the effectives of the seals around the outer perimeter of the frame and the masonry.

The popular saying "Theres no accounting for some folks". Some make decisions completely blindly while others pursue a firm understanding of every detail to amazing degrees of accuracy before making no decision at all.