How much a webmaster will charge to maintain a website will vary from webmaster to webmaster and how much and what kind of work the webmaster needs to do to keep the website maintained.
If the webmaster has to do programming in the background then you might be charged $100.00 an hour.
If they need to just fix a few things and remove spam from comments, or forum posts etc then you might pay $10.00 an hour.
Other things such as changing the design or backing up the website might run $30.00 or $40.00 an hour.
But again there's no really correct answer as it will depend on the job and what the webmaster has to do to maintain the website.