Generally, about $100/hr. Be sure to get Commercial Vehicle and General Liability insurance.
Seriously, there is a lot more to it than that. What type of plowing do you expect to be doing? Driveways? Parking lots? Are you in a rural or suburban/urban setting? How much experience do you have (have you ever plowed snow)? Some charge by the hour and type/size of equipment. Some per plow by depth. Some by the season. Most have a mixure of these. Is there a chance you could sub for another plower the first winter?