Providing he does a good job; i.e. you can't see the fixings, all mitres are clean cut and lines are perfectly parallel to stairs and skirtings; �140 would be more than fair and very reasonable. ( Cheap I would say.)
Hall ,stairs and landing are quite an area, even in a very small terraced house. Don't Know what you have, but it will certainly improve the look of the the areas concerned and when decorated will really add a touch of class.
As theonlyone has said " Go for it and let us know the end result."
Best wishes.....Ron.