I call this the guesstimation process. Whatever result you come up with you have to make the judgement call of whether you think you can do the job in the time you have estimated.
1) Try and work out the square metreage of the area of glass you are proposing to clean. If any of the windows are less than a metre but you would have to move your feet or ladder to clean them treat them as if they were a metre square. It will take you a minute to actually clean each metre of glass.
2) Will your WFP partner be able to reach all inacessible areas or will you need powered access? If you do need a cherry picker you have the hire cost + your handling charge + delivery etc and need to think about permits to use it if you are going to be on roads, certification (IPAF) to operate it and where you will store it overnight (perhaps). If you are using it internally get the hire company to survey the job as consideration has to be given to floor loads.
3) What else are you cleaning? - Frames? signage? Cladding? For all of these you will have to make your own time estimate of how long it will take you to clean it.
4) Finally, take all your cleaning times, double it then multiply it by your hourly rate then add on all hire costs including your handling charge. Add 5% to the whole lot and quote that as your price but offer 5% discount for payment within 14 days. You will then get paid in 30 ;-)
Why double the cleaning time? Because that's what it is.. the cleaning time. You will spend time and effort putting cones out if it's daytime and Joe Public is about, if it's early/late in the day you want a premium, you spend time accessing the work and you spend time getting it inspected and signed off.
Finally whatever price you come up with (excluding the hire costs) just look at the job and ask yourself if your comfortable with the price before you give it to the customer. Don't get downhearted if it takes you longer to do than estimated the first time. Practice makes perfect and you will do it faster the next time.
Best of luck. Phil D