It's depends on your quality service. Usually for normal Seo $200/month is enough but for Seo specialization like Seo audit, target your particular keywords then enterprise package will be run, which is approx $1000/month. Medium level Seo which is for small entrepreneurs and business $500/month which is include all Seo strategies such as keywords ranking, monthly Seo reports, web optimization etc.
But I personally suggest that to do Seo by itself. Because there is nothing like scary things. Just you need to put your concentration. You also can join our Seo training program.