Freelance Maintenance Pricing Ideas

Hi /r/webdev!

I'm working on setting up infrastructure to start developing freelance for small businesses and organizations locally.

Can anyone chime in with ideas on pricing schemes for maintenance on small websites that I manage? Monthly fees seem to make the most sense but would need to be tailored to the level of maintenance that a site would require. Are there other ways to fairly price out maintenance work?

Thanks for your help!