The first thing to consider when asking yourself, how much does SEO cost for my small business? The average monthly retainer is $500 to $175 for one SEO agency, but the amount will vary from month to month. A one-time fee is also possible, however this should not be used as a last resort. Monthly retainers can be the best option for small businesses. Monthly fees are also very flexible. You can adjust your monthly payments depending on your budget.