Companies continue to make money because Google and Bing place them at the top of search results. It can be more cost-effective to hire an SEO professional when you need technical SEO tasks at some point in a project. As I mentioned earlier, your initial SEO cost is likely to be higher than what you pay if you opt for monthly SEO pricing plans. If you decide to hire an SEO agency or consultant, ask them if they think SEO will pay off for you.
SEO is a medium-long term strategy and can be very worthwhile if the company that manages your SEO knows what they are doing. They'll also know what tactics to avoid, such as black hat SEO techniques that could lead to penalties, making all the money you've invested in your SEO strategy worthless.