Try explain that price per square metre is a very vague pricing structure, as it doesnt take into account the terrain or height , condition of windows, or the method needed to clean them ie wfp, trad, cradle, abseil etc
Try tell them you will give a free quote and go see the buiding, or alternatively go look without them knowing and then give them a price per window ??
But unless its runs of easy ground/first floor windows, i wouldnt give a price per sq.metre, if it is easy then go for £1 or there abouts, depending on how much there is, if theres loads of easy to clean glass go in cheaper, depends on the frequency of the cleans too....And how far you have to travel etc etc etc
Go and quote it is easiest
I know that isnt the answer you were after but hope it helps, i always find commercial quotes a steap learning curve and eventually you were land more and more as you understand it more, im still learning but it gets easier every time