There are many answers to that question. Before anyone can give you an estimate are is alot of info they need to know. Where does she live? What time of day does she expect the driveway to be plowed by? Does it include any hand shoveling? Not only how long is her driveway, but how wide is it? Is there room for the plowed snow to go to?
And then you need to know that when someone gives you the price per season, they have to estimate their projected costs. Such as fuel increases, how bad was last winter and did they loose their shirt by not estimating high enough?
Last but in my opinion this is very important-how reliable are they and how good of a job do they do? Then ask around in the area where your mom lives and get estimates and recommendations from others who pay to get their plowing done.
Good Luck,
Kat