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according to SEOmoz

• Hourly Consulting
  The simplest way to price a project is to charge by the hour. Rates in SEO vary with the lowest, entry level folks around $40-50, mid-tier consultants around $100-$200 and high-demand firms & people from $300-500.
• Project-Based Consulting
  Many SEOs will use the business model common to web development agencies and charge a flat fee (often in several chunks over the course of a project). The total price is based on an estimate of time, effort and personel involved in the project.
• Contract Services
  Plenty of SEOs offer dozens of individual services, such as Debra Mastaler’s directory submission service, Eric Ward’s link building & publicity campaigns or Jessie Strichiola’s SEO assessment services. These offerings present a single price for a set amount of work.
• Standard Profit Sharing
  Some limited number of SEO providers offer profit sharing based compensation. These frequently include a relatively small down payment to begin work and then a percentage of revenues (usually before non-essential expenses) from sales through the website. This can be a good option for SEOs who have great confidence in their abilities and are ready to assume a significant share of risk.
• Modified Profit Sharing
  As above, but modified profit sharing typically includes a clause that gives the SEO firm a cut prior to any expenses and may even set minimums of payment. Other modifications could make the deal similar to a Pay-Per-Action (PPA) or Pay-Per-Lead, the latter of which can be a better way to limit risk. Both SEOs and those seeking their services should be wary of any kind of profit-sharing deal. It’s akin to a real business partnership in many ways and shouldn’t be treated with any less weight.
• Monthly Retainer
  A few good SEO firms I know use a basic monthly retainer with a standard workload package.
• Pay for Rankings
  This is one of the more interesting strategies that SEOs employ. The idea being that you pay one price for reaching, say, page 2 of a particular result, another price for position 10, 9, 8 and so on, usually with particular bonuses for #1-3 rankings. It really only makes sense for companies seeking to rank for a particular set of terms/phrases that they know converts quite highly.
• Pay for Traffic
  As with pay-per-ranking, a traffic payment system treats SEO very much like PPC. I like this model in some respects, because it does measure the SEO’s work, but it can get messy as the quality of traffic isn’t measured here (of course, this usually only counts search engine traffic, but still…).

With those covered, let’s dive into the prices themselves. If you’re in the market for SEO services, there’s a lot of factors that can affect what you should expect to pay, not to mention the great number of formats an SEO contract might take (as noted above). The following can all affect the price you might pay for organic optimization consulting or hands-on services:

• Size & Complexity of Website
• Size of Brand/Organization
• Difficulty of Project / Competitiveness of Rankings
• Personality Issues (if you’re a very controlling personality, expect a generally higher price)
• Reputation, Notability & Demand for the SEO Firm

The prices themselves are all across the board, but here’s my take on the general ranges: