That is the hard thing about snow removal. I have customers on flat monthly rates for snow removal based on an average for the last 3 years of snow. Yes, if we get a lot more snow than usual then the customers wins, but if we have a dryer winter, then I win. It's a total gamble!
What I would do is think about what you would charge the places individually if they were regular residential customers. So example, 12 drives, at $25 each, then you would make $300 for those 12 drives if doing seperately. Once you figure that out, I feel that you can come to a better conclusion on everything