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The larger the site and the more different kinds of areas to look through such as profile pages, blogs, forums, products, categories, etc will be more expensive as you have to check each for different types of problems.  You can probably glean a few improvements you could make yourself from SEO audit articles, our most recent is here: http://www.theedesign.com/blog/2014/seo-audit-design-doesnt-bring-you-traffic Then of course there is: http://moz.com/blog/how-to-perform-the-worlds-greatest-seo-audit