The single biggest cost in most loft conversions is the structural work: strengthening or replacing the floor so it can carry a habitable room, and forming any dormer or roof alteration. This is skilled work that often needs steel beams and a structural engineer, and it is not an area to cut corners.
The staircase is the next significant item. A loft stair has to meet building regulations for rise, going and headroom while fitting into a limited footprint, so it frequently has to be made to measure rather than bought as a standard flight. A bespoke staircase costs more than an off-the-shelf one, but it is often the only way to make the space work.
After the structure and stair, the costs that add up are the dormer or roof windows, insulation to current standards, and the second-fix finishing: doors, skirting, flooring, and fitted eaves storage. Plumbing and electrics for a new ensuite, if you add one, also push the figure up.
Because every loft is different, the honest answer is that the biggest cost depends on your roof. A simple rooflight conversion in a roof with good head height is far cheaper than a full dormer with a new staircase and ensuite. A measured survey is the only way to get a reliable figure.