in practice, and that's to charge an hourly rate for work on the server. Unless you approach it in that way you'll find it impossible to predict your costs, and consequently your charges to the client.
That's the way I work with all my clients, I tell them how much an hour's work will cost them, and they are billed accordingly, after the updates are done. I advise them to collect their amendments and tweaks into batches, so they get better value for their money. There's a minimum one hour charge, and it doesn't make sense for them to pay for a full hour just to alter the date on something, or to replace an image.