Can't advise you on what to charge because I don't know what "very large" is...
If the marquee is a plastic coated canvass type then g101 will remove green algae easily, if it's tree sap and black then the job will take longer.
If it's got an open fabric weave then I'd use a sodium hypochlorite solution
But test an area first.
We cleaned 7 'witches hats' last week at a school each approx 7 meters sq. and 3 meters tall, these had a lot of built up grime and took 2 men around 4 hours to do
Darran