You could always try guesstimating a quarterly cost for minor upgrades (minor text changes, what have you) throughout the quarter. You'd want to clearly define:
* what is a minor upgrade (covered by this bulk payment) versus a major change (would be a separate charge)
* the maximum amount of minor changes per quarter - once they've hit this number of changes per quarter, they could either pay for additional changes on a per-change basis, or purchase another "block" of minor changes for that quarter
* if your client has a quarter where they pay for this block of changes and they only ask for two minor changes that quarter - will you roll over any of their payment to another quarter? (If they're asking to pay "in bulk", I would not allow this option. Either they pay in bulk to have you "on call", or they pay as they go for the changes they need made.
As long as you define things clearly enough - so that both you and your client know exactly what to expect - you should be good. Make sure that it's something you can live with, though. Don't get yourself into a situation where you've defined something for a client and you come to resent them for it.
I would not count creating new pages or graphics as a "minor change". Removing a paragraph (or changing the content of a data table or bullet list), adding a sentence, changing an email address or phone number...those are "minor changes". Adding a new page, replacing all content on an existing page, adding a large data table, creating a new graphic...those are right on the borderline. If the table was two columns and two rows, with a number or a few words in each, that's kind of minor. If it's a larger data table...that's significant work. (Essentially: if a task is going to take you longer than 30 minutes, including uploading files, it's not a minor change.)