You will need to work closely with your client and find out how much they can afford. You can charge an hourly rate and be paid by your improvements to the site. This way a person can see how you work and test the improvements that you make to the website. Making small improvements over time is a good way to work and help out a website owner. If you want to redo the complete site then you will need to decide how much your time is worth to you and what value you can offer the person for the amount of money you are charging.
I would suggest that you make some custom themes for WP and sell them. You can make a lot of money like this and people love to purchase themes to make it easy for them to design their website. You can start off by showing your work with one or two free themes and then move up to paid themes.