A lot will depend on whether they will let you change the design of the site, so you can use a premade template and just adjust it, or whether they want exactly the same look, which means you will need to build a custom template for them. That would increase the cost quite a bit.
How long do you believe it will take to complete the project?
What do you believe is a fare hourly rate given your experience and/or lack thereof?
Are you competent in both front-end and back-end development?
Have you worked with Joomla before and if so to what capacity ie. theming (htm/css) and/or feature enhancements, bug fixing etc. that require programming?
Is the client familiar enough with Joomla to be happy enough with how it works out of the box or will custom development be necessary for certain features putting aside theming?