Don't forget to factor in the time for removing posters, paint, blue tac, selotape etc from the windows and all the stuff from the sills to actually reach the windows.
You would be better off on day rate + a bit, than £ per window.
saying that, I have just completed one, on my own 6 days, £3k so they can be worth it if you get it right and you don't mind hard graft, cos generally thats what they are. usually filthy aswell.