Okay, I'm thinking of uPVC frames, casement windows with the 2 side
parts of the bay having top openers and side openers for the bottom
part and I want it fitted.

Well my parents had a HUGE bay done in quality hardwood (may have been oak,
not sure). Each window was an opening sash. They were double glazed,
draughtproofed and factory coated in acrylic. They came with a 100 year
guarantee. Cost was around 3000 quid fitted very professionally. Your
original quote seems very high unless they were factoring in structural
repairs to the bay.

BTW, you may find that the window frames are structural. Replacement with
uPVC may result in collapse.

Christian.