I think the prices vary with plasterer to plasterer. I had a bedroom done which is about 12ft x 9ft, had the ceiling plasterboarded and walls skimmed. Then had the stair area done which was a straight run up to the first floor with new plasterboard on the lowered ceiling. I paid about £1500 for that about 6 years ago. The lad who done it done a fantastic job so I didnt mind paying out for it.