There are many factors to consider , how high is it ,access, frequency, type of window wood,upvc,where are the vents, how big are the pains of glass /how many are there , how long will it take you , etc it’s a personal thing what you think is a fair price others would say to cheap or to expensive work out what you want to earn per hour guess how long it will take and to some extent faster into your price what kit you will need if using a 60 foot pole it should be dearer than using a 25 foot pole as replacement costs will be higher and it’s harder work , less people will have the kit to do it so charge accordingly