It all depends on how many square meters the job has total. There is no such thing as a price per square meter period as cleaning one square meter of glass will work out more expensive than cleaning 4000 square mtrs per square meter.
It is common to price per square meter for larger jobs.
I would love to earn £1.50 a square meter I would have retired by now LOL
So you need to know how many square meters in total, then work out how many man hours is required to complete the job, what time frame the job must be completed in, then add your running costs.
Its a simple equation if you know these figures.