Once the position is settled, the price is shaped by choices you can see and choices you cannot. A bigger opening needs a bigger unit and more framing. Glazing that keeps more heat in costs more than basic glazing and earns it back across winters. Opening or fixed matters too, because a landing with a window you can open is a landing that can lose stale air.
The thermal side is not a matter of taste. Part L 2021 treats the two products differently. For a rooflight the figure is 2.2 W/m²K. A roof window is judged as a window instead, so it needs 1.4 W/m²K, or else Window Energy Rating band B. Your quote names the category your unit falls into. A misted older unit often has glass that no longer insulates at all, so replacing it makes the landing warmer as well as lighter.
A tight, well-flashed frame matters more than a fancy finish, and we put the money there first.
One more choice sits in the glass itself. A unit with a tinted or solar-control coating cuts glare on a bright day, which matters on a south-facing slope over a bedroom, while a clear unit gives the brightest light on a shaded landing. We settle it on site, standing under the spot, because the right answer depends on which way the slope faces and what you do in that place.