break it down into sections and then add it all up,
walk around the building as if you would clean it,add up how long it would take to clean it all around then add another couple of hours to play with.
work out what your day rate would be,the minimum you want to work a day and work out a price from there.
this is generally for big jobs.
walk around it at least twice and look at the state of the windows and maybe even clean 1 to see how they come up on a 1st clean.
easy really just use your common sense ;-)
regards