I'd charge about how long it would take to comb the area with an ATV or something similar. If you are responsible for the entire area, if there's trash or not, you'll still have to check for it, won't you? SO I'd say ride around it once, and see how long it takes, and bill that by the hour for each day.
Never had this large a property, or this type of work (trash collection) so this is just how I think I'd handle it.
Good luck with it.
Just kidding, your question is much too vague, you must tell us how much trash, what sort of property it is, etc. Is it a parking lot, all paved, is it the side of a cliff, where people throw a lot of cans, etc....?