Average cost of SEO?

How much would SEO run for an average website done by a firm? I know it varies, but what factors into the cost? (like making one website more expensive to do than another?)

There isn't really an average cost, it depends on how much work you want to company to do. Generally you get charged an hourly rate for any work and sometimes an extra amount for every quality, relevant link back to your site. Writing keyword rich meta tags and content for your site can be time consuming so a lot of time is spent on that and then obviously it takes time to submit the site to relevant search engines and directories.