I think you need to look at what the customer wants, i.e. frequency etc... then when you know what the requirements are you then base your price not on how many windows they have but on how long it will take you/hourly rate.
It has been mentioned already, look at the benefits - early morning, wet weather work, commercial, looks an easy job and already we have some on here saying £150 and some saying £80??
My advice, don't be greedy just because this is commercial, yes they generally do pay more but they are just as likely to accept a cheaper quote to save money. Lets face it, the fact that commercial offices have their windows clean is based on HR requirements and not the desire of the MD in most cases.