According to Search Engine Watch, the majority of businesses pay anywhere from $2,500 to $5,000 per month for ongoing SEO services.  However, this figure is highly variable and can be far higher or lower depending on a number of different factors. Here are just a few of the considerations that come into play when determining pricing:
1. The size of the business. Larger businesses usually have more extensive ongoing SEO needs than smaller businesses. This translates into a larger workload for the SEO firm, and thus, a larger monthly fee.
2. The competition of the industry. Trying to rank in an extremely competitive field takes a lot more work than trying to rank in an industry where there are only a handful of businesses competing for the top spots in the search results.
3. The level of support provided by the SEO firm. If you are paying a low monthly fee, you usually won't get the same level of support and contact from your SEO team as you would if you were paying a higher monthly fee. Monthly SEO packages that are priced under $1,000 usually only include the most basic services, such as link building and monthly status reports. Packages in the $5,000 and up range, on the other hand, usually provide access to a dedicated team of SEO professionals including web development specialists, copy editors and link building experts. You will usually also receive detailed analytics and ongoing support. 
Most experts recommend avoiding SEO firms that charge less than $500 per month. Oftentimes, these low-cost options can wind up doing more harm than good. Their link building is usually outsourced or is performed using automated methods, which can wind up getting your site penalized by the search engines. They may also outsource content creation to overseas providers, which often results in low-quality articles that are difficult to read. 
A good rule of thumb is that small to medium sized businesses should expect to pay anywhere from $1,000 to $5,000 per month for ongoing SEO services. Larger businesses, on the other hand, may need to budget anywhere from $5,000 to $10,000 or more per month to meet their ongoing SEO needs.
This video takes a closer look at what SEO services cost, including which factors come into play when determining pricing: