Window costs depend on many factors. Saying a 1m2 window costs this much only works for a standard house/window/.... Nearby there is a (as my crystal ball suspects) similar house; the entire south gable consists (almost) only of very large double-glazed wooden elements, large roof overhang, extremely large lift-and-slide doors, exterior roller shutters with motors. The house was built in 1978; back then, the best of the best and optimal insulation far better than what was common until the 1990s was installed. The owner still lives in his dream house, everything still original, kitchen, bathroom, windows. If something breaks, it is repaired or if it needs to be new, it is brought to the best possible state of technology. About 15 years ago, 4 large glass panes (= about 1/3 of the glass surface of the house) were replaced with modern double glazing because 3 of 4 were "blind." Two years later, 1 roller shutter motor was defective. 4x motors new + insulation of the roller shutter box. 4 years later, oil heating total failure, 1x new oil heating, the best that could be bought.
Replacing the 4 panes cost about €16,000 fifteen years ago. The glass was expensive (size) but only accounts for about 1/3 of the costs. The biggest cost driver was how to get the panes there without destroying the (large) natural garden (owner's requirement).
In terms of annual savings, it will not pay off, but it had to be done "anyway." According to the owner, heating oil demand has dropped by about 1/3... there are no exact data on what saved how much. The large oil underground tank under the garden is only refilled every 2-5 years depending on need or oil price.