I do a bunch of commercial and residential for that client and my sidewalk crew does the walks, so it's based on that. It gets billed at 10 minutes, which is one 6th of what I estimate my hourly.
If you can come up with a minimum, based on what you need to make, you can go that route.
Hope that helps.
If you have several drives in close proximity, you can do them for less.
Example, you sign contracts for a combination of 10 driveways, lots, etc and the route takes you say 5 hours including drive time. You have to Cover all your expenses, repairs, wear and tear etc and pay yourself what you're worth for a day.
I'll also add that the price increases with snow depth.
Ask your buddy how much he'd pay you as a sub and tack on some more.
For your 1st year, don't take on more than what you think you can do in 4,or 5 hours. Traffic, weather, etc all add time.
There is no magic number.