Newer SEOs uncertain about how to charge for their services may find this article helpful: How Should You Price Your SEO Projects?
Author John Britios (known on this forum and others as Webnauts) doesn't get into exact numbers, but talks about how to analyze a job's intricacy and competitiveness. For example:Lay out a plan, identifying the ultimate and intermediate goals, and the tasks to be undertaken to get you there. Establish milestones by which you can gauge your progress. You should do this for your own SEO efforts, as well as for the benefit of your clients. Examining those areas in which you failed to achieve an intermediate goal or milestone will afford you insight as to the causes.
Explain clearly to your client what the intended tasks, goals and milestones are to be, and by what metric they’ll be measured. Begin the actual project with a written agreement, agreed to by both parties, that fully describes the responsibilities of each party, so as to avoid conflicts later.
John also references a SEOMoz article: SEO Pricing & Costs - What Should You Charge / How Much Should You Pay?Off-page SEO costs depend more upon keyword competition, which means that you may be incurring additional expenses when chasing the more competitive terms. It is common for SEO companies to price various aspects of off-page SEO, by unit, such as a given dollar amount per inbound link from a blog, article or press release, or a directory listing. This allows you to bill incrementally, and easily adjust for changing requirements.
How do you analyze and price a project?