I'm soft; I would've done what FTP did; bit the bullet and cleaned it. I don't usually do the 'first clean extra charge thing'; unless it's really justified.
However, I have learnt when quoting for stuff to take my time and really look at each window; it's height, access, the frames; everything. I almost imagine myself doing the job when I'm quoting.
A cursory look at a property followed by a price is just asking for trouble.