How much should I charge for maintaining a simple WP site per month?

As a freelancer most of my clients want to maintain their wordpress sites themselves, but recently I’ve got a few clients who wants me to maintain their site. This includes keeping everything updated and safe and doing regular backups.

My plan is to use InfiniteWP to monitor all my WP sites, and set up automatic backups to a cloud service.

But the problem is how much I should charge for this per month(per site)? Is my hourly rate per month a starting point..?

If you’re a freelancer maintaining wordpress sites, how much do you charge? ??