Hawaii Cleaning Co.

Aloha from Hawaii, I own a cleaning CO. in Hawaii and have been asked to provide a commercial bid for a concrete (sidewalks) and Parking lot pressure washing. This is for a large Mall. The sidewalks have not been pressure washed in a few years and are almost black with dirt and mold, the asphalt needs to be cleaned for re-striping. My question is, does anyone have any idea if I should bid this by the sq. ft?., and if so , how much?, approx 50,000 sq ft. of each (concrete and asphalt). And, or what, are some of the pricing differences you charge for concrete and asphalt pressure washing and cleaning? Once the initial pressure washing is completed, what do you think would be a fair (sq. ft?) price for a quarterly maintenance pressure wash. Also do you have any recommendations for various chemicals that may help in the cleaning. Im stumped on this one and don't want to get stuck on the low end. However, I'm not quite sure what the low end is out here as there aren't any pressure-wash specific companies out here. Thank you very much for your help!! James