I am mostly in the same boat as you, so I'll offer my experience.
First, what kind of work would you be expecting to cover with the maintenance agreement? Are you going to fix their phones? Are you going to fix stolen equipment, or Domain Admins who have ran wild with the DC? Are you going to switch out all their computers on a set schedule? Or are you going to cover things like minor virus infections, back up restoration, new user onboarding?
You'll need to determine the kind of work, then estimate (as best you can) the cost per user to manage their equipment and requirements each month. User account maintenance should be easy, less than 1 billable hour each if your system is setup correctly, as an example. Teaching them how to access company resources (i.e. printers, Intranet sites, server shares, etc) should be less than 1 billable hour if the user is teachable.
Now, why am I saying to do it per user? Because work sometimes scales, it sometimes doesn't! If you have 2 users at each site, and 3 sites to manage, you can easily cover the sites together, because there is a small user base. If one site grows to 50 users, another to 75, and the third to 10, that becomes a lot more work, even if things do not break.
When quoting a price, ensure you aim for the higher end so that you can start offering discounts or downgrades to meet their budget. And make it very clear that anything outside of the general maintenance tasks you are offering is over and above from xx hours of work.
Your mileage will vary, but you are essentially turning into an "on call" kind of freelancer, who may be required to fix issues no matter what time of day, and that is going to cost the client money for that kind of accessibility.
