What drives up cost for porta potty rental in remote areas?

I got a quote for a single standard unit for a 3-month hunting season at my camp, 50 miles out of Bozeman. The ground is a rough dirt clearing and the service truck can reach it, but the drive is long. The quote includes a delivery fee, a monthly rental rate and a service fee for weekly pumping. The total seems high. I know remote placement adds cost, but I want to understand exactly what drives it up. Is it all the fuel for the vacuum truck? Does the rough ground affect the price? What should I look for in the quote breakdown?