I don't think you'll get a "catch-all" amount to charge, but there are some things to consider.
First, you'll want to ask your client how much they want to do in maintaining the site. Some clients, even with limited knowledge, will want a lot, but some will still want you as the professional to keep it up. If they want a backend to allow them to make changes, then you can charge them for that set up and turn 'em lose.
If they want to keep you as maintaining their site, you'll need to find out about how much change this will require on some kind of time basis (like daily, weekly, monthly, changes). Is the site somewhat static, or will it be dynamic with lots of changes?
Then you will need to determine what you think will be the amount of time you'll have to spend on the site to maintain it. Some clients want a hourly rate, while some prefer to have a monthly charge. Either way, you'll want to set the amount at a price that they will happily pay, and you don't lose money due to excessive time spent on it.
My personal likes for maintenance is to have a set monthly fee, but only to do a certain amount of changes in that time period.
The biggest thing is to know your clients, know how much time you can spend on their site, and determine the price that is satisfactory to both of you.
It's not a cut-n-dried answer, but I hope that helps you when setting up a maintenance plan.
Dave
