There are basically two types of SEO outfits. There are the "chop shops" of the world offering dated solutions like link building, etc and charging a monthly fee for supposed maintenance. Then there are credible experts who understand that the overall experience of the web assets is what catapults KPI's. One of the most important factors that's widely overlooked is technical compliance which experts adhere to and follow closer than a CPA follows tax law changes. Having said all that, ultimately, you get what you pay for. A proper outfit will charge you an appropriate fee to get your assets optimized. They will rarely charge you for monthly "maintenance" and instead may offer additional experiential suggestions for the brand. My final bit of advice is budgeting for this service so that you're not stuck learning and guessing trying to reinvent the wheel. Many try and eventually hire an expert.