I do this all of the time...
1. evaluate the site to make sure it can be ranked due to not being in a dead market or crap site in the first place.
2. quote based on the specs of:
$ what you have to do to his site to get it ranked; seo, changes on and off site.
$ time investment you are going to have to make to get this accomplished.
$ time investment of how big of a window it is going to time for these changes to be made and take a positive resulting effect.
$ what tools, skills, resources, outsources, etc. you are going to need if any; article writers, etc.
As well as anything else you can think of that comes into play for this unique project.
Once you have all of that information, then decide what your time is worth to you based on what you do within an average hour and what effect/impact it will have on the end result.
I charge different custom rates, depending on what needs to be done but have a standard price that I won't budge on as the min. Also, get at least half upfront and NEVER work for free unless you are doing it for the point of proof that you need to gain the first project. IF you have to setup a site for free to prove that you can do it, which you should have already done this to know if you are up to snuff or not, do it on your own site projects. Work for free once and you will never get paid. Your own projects are sure to pay you down the line.