you have to check how much concrete is left on the windows normally a house that would take me half an hour to clean say a £10-£12 house id charge 5-10 times the price depending how much scraping is involved,youll probably find house will take at least 5 times longer to clean on a builders clean.
p.s check to see what state the frames are in also as this is more work again.
hate builders cleans i find them soul destroying.