You set yourself a realistic price per hour you would like to earn.
When you go to price a job, estimate how long it will take you clean the building including time to take ladders on and off the vehicle and charge according to your price per hour.
eg you set youself £20 per hour and job takes 30 mins then charge £10.
You might also want to have a minimum price as some jobs wouldnt be worth turning up to clean.