I basically charge 10% of the project price as a yearly fee.
So if I charged $3,000 for building the site, I would charge $300 per year for maintenance, but I have a minimum fee of $100 per year for lower priced sites.
(I have done a few charity sites also, but I still charge for them, I don't do anything for free except my own site)
Minor changes = change/add/delete email addresses, update to a text file (I use either SSI or PHP includes for content sent to me by my clients making it very easy to change), etc.
If they want to add new pages then I charge an hourly fee depending on what the pages need. My hourly fees range from $60 to $140 depending on the complexity of the pages (static vs database, etc.)
Mike...