I agree with DevelopmentExec in regards to first figuring out how much per hour you want to make at writing, then estimating how much time it will take to complete the project and turning that into a per word amount. Here's what I will add:

1) Take your time estimate and multiply it by 1.5. If you think it will take 80 hours, then make your estimates around 120 hours. This is what I used to do for programming. Projects usually take longer than you think because there is always something that comes up, and until you've done this over and over again, you probably won't nail the time estimate.
2) Be very clear in what you will be providing. For example: 1 creative pass with content writing and 1 editing pass with $x extra for each additional editing pass. (Or however you want to break down the work -- just be clear on what your responsibilities are going to be.)
3) Get some money up front. This is a different sort of freelancing than taking a contract from a publisher. I either like to break down a project with 1/2 to start and 1/2 at finish for smaller projects or 1/3 at start 1/3 at middle and 1/3 at end for larger projects. This makes sure if the person gets into it then decides its more work than they imagined they won't completely stiff you.