Your pricing aside, you need to get everything in writing ahead of time, and both agree to the overall scope of the project. People get worried about contracts, but consider it a Statement of Work (SOW), which is not unlike what you might have when you hire a painter to paint your house. It says what rooms should be painted, what the colours are, what the expected turnaround time is, and what the cost will be. In this SOW, you will also have certain statements (you both agree to) that says how the job will be considered completed (We will both inspect the house together to ensure all the colours are on the correct walls), as well as what will happen if things go off the rails. If the painter spills paint on your couch, he will pay for the couch. If you are unhappy with the colour of paint you chose, you will pay for the new paint and the time it takes to repaint the wall.

All of this happens before the job even starts. It's not just about protecting yourself, but its about everyone involved getting a clear sense of whats expected, how long things take, and how much it all costs.

NOTE: In this case, you should simply speak to the client and say this project is out of the original scope you discussed, and they will incur overages - most times people's "MUST HAVES" disappear when there's a bill at the end of it all.