Take the lot and fiqure out how much time it will take you to clear the snow off the lot 2" increments. Then take you hourly rate and mutilply the two. Add a fudge factor, and that should get you started.
Or do the following. Lets say I have a lot that I charge 100.00 to plow every 4". I divide the 100.00 by 4" and I get a per inch price of $25.00. So if we recieve a 10" snow fall I would charge 250.00 to plow the lot.