We took out sash windows and put in 'normal' hinged UPVC (In southeast for reference). From memory it was ~£800 per window (lounge bay windows) and ~£600 for a rear door. Smaller windows about 60 * 100cm were ~£400.
Our sills & casings were in good shape so it would have been more if they had needed to replace those as well, but they checked when they came to give the quote.
Might vary from different installers, but they were a bit rough and left a few dings in the casings so we had to sand it down, make good with polyfiller and paint. All doable but would have been much easier to sand flat an empty casing instead of around the window so I'd happily have paid a bit more for a company who would also make good the painting/after work.