We had our original 1930s sash windows restored, involving removing the sashes, replacing any unsound wood (mostly sills), fitting draught seals, replacing pulleys, cords and other hardware and repainting from bare wood. I think we paid around £9,000 for 15 sash windows, but this was five years ago.
I don't think "Eventually they start to rot" is really an argument against timber windows. Yes, there's some ongoing maintenance, but only touching the paint up every couple of years and probably getting them fully repainted once every 7-10 years, but with that they will last indefinitely. They've already provided 90 years' service!
The one remaining window we didn't have refurbished is the stained glass landing window on the side elevation. We left this when we did the others as we weren't sure if we would have it removed when we extended. It turns out it'll still be there, but difficult to access for maintenance.
For this one window, we've had a quote for a uPVC replacement incorporating the original stained glass encapsulated in new double glazed units. That's going to be £1200. We had another company quote £2500, so shop around and be sure to get detailed specs for work so you know what's included.