I've done this type of thing recently and you would not believe how much of a pain in the ass they are. To get every bit of paint off means going over again and again with a scraper - and you have to be very careful with a scraper too - which takes even longer. I price these types of jobs at least 5-6 times normal clean price - often more if really bad. Otherwise, once you start doing them you will realise they take a lot longer than you think and really regret underpricing the job.
I have not done a few where they didn't want to pay my price and I wouldn't reduce because I knew how much hassle they would be. Sometimes it is better to walk away; one recent one wouldn't pay my quoted price so I put them onto a tradder I know - he priced it (I did warn him!) and got the job but has never let me forget how much longer it took and how much he lost on the job - he thought 2 hours and it actually took him from 8 am till 6 pm.
Also, a little tip when quoting - walk round with the gaffer and inspect every single window as best you can for:
1. Amounts of paint (will give you an idea of time needed).
2. Any existing marks/scratches on the glass ( to cover your backside).
3. Any plaster/cement splashes (cement, in particular, is the devils work to remove and the removal itself can cause severe scratching if not done carefully).
4. Grease deposits from putty - a real pain too!
5. Stickers - again can be a nightmare.
Just my 2 cents
