$20/hour and up, but it's more reasonable to have a contract that bills based on property size and event type. For the small walkway/alley type properties my way, I charge $5 for a dusting or salting, $15 for 1-6" and $25 for larger events. The effective hourly rate ends up being about $40/hour.
Not sure what professional contractors charge, though obviously that's all over the map depending on whether it's a guy in a truck versus true professionals like my brother's company. There the profit is all in salting, plowing is typically light profit, but they often lose money when they need to pay $20+ hourly labor to clear sidewalks and stuff.