Any idea what dormer windows cost?

[Madmog]

Thinking about buying a property, but would want to put dormer windows across the back of the property. They would need to cover two bedrooms with internal lengths of 12.5' and 8.8'. There are presently velux windows, but only enough head height to put in one wardrobe in one room, after you've excluded certain walls for the beds, radiators etc.

Guess we would need planning permission as well, but as 30% of the property in that style have dormers, I don't think it would be a problem.