Josh, Frist Iwould ask myself why in the hell would this customer want a 30 foot X 20 inch gravel parking area? HAHAHA! Really you need to figure out time& cost of material. Is the material being hauled in? If so, can the truck get close to the shed or will they have to dump out front and you move it all back to the site? Do you have a place to dump the spoils (dirt you take out)? Will you be doing solo or have help that will need to be paid.......Ouch my brain is hurting now!