Just had mine done as part of a house refurb + extension, hard to put a price on it as it was part of a much larger project but I think somwehere in the region of mid 40’s.
Still have to get flooring and blinds for the velux windows, oh and bars to go across the window opening to get our building cert. We went for big tilt and turn windows rather than a juliet balcony but the building inspector insists on having something to stop people falling out of window – probably correctly.
Ours has a dormer, a hip to gable, two velux and am ensuite-ish bathroom. We also chose to remove the chimney stack in the roof (and throughout the house)
We had to sacrifice a big chunk of our already small bathroom to get the stair turn in so we turned that into a wet-room.
If you have any questions please feel free to ask