if you are going to be a solid SEO you understand what you can do, how you can do it and the time it will take you to do it. You will also know how to run an analysis to forcast the outcome of your sites success. Based on all the initial information you recieve you can base your cost per project instead of per hour. You really need to understand yourself and what its worth to a company. How much revenue will a company make from you doing SEO on their site. Will they profit in a way that will make them millions? thousands? hundreds? depending on the industry you are trying to nock down with your site being nocked in how effective will your work be, how long will it take you and what kind of strategy do you need to put into affect. Do not under estimate your skills and present them in a way that will make sense to a company. Many companies have reason these days to be careful about who they hire. Lots of SEO's say they can do things but many of them just say it because there are not many educational credentials that can be attached to ones name. The credentials that get attached to somebodies name are those of previous sites to show a proven track record. Hope this helps. Enjoy pricing yourself.
