Chris,
very large job. are you doing this single handed. if so would sub out part of it. for larger jobs i sometimes price based on a day rate, and estimating how long it will take, plus materials, travel , hire cost etc if required.square meterage can be misleading as you know, a large area with easy access and drainage and easy access to good water supply is often easier and quicker than a really small difficult site requiring bowsers and pumping out etc. so lots to consider
Good Luck
Ged