Builders cleans are always worse than you think they are.
I charge at least triple and often more. If the person doesn't accept my price then I walk away. Before quoting I have a very carefull look at every single window inside and out and I even get the ladders off to do that if needed.
It's also important to find out exactly what the person expects because if they want every little tiny last bit of plaster/paint/cement cleaned from the frames then one window can take an hour or more.
If you start the job and then realise you're onto a hiding for nothing, be prepared to say sorry, can't do the job and do a runner.
Also as per above, a scratch disclaimer is a must.