If you are interested in how much you should pay for SEO it may be useful to analyse SEO companies, their structure and their techniques. Perhaps the techniques they use to market to you are interesting as well.
Discovery is very important. Say I have 20+ Gold Coast SEO clients and 17 websites I own myself, that is roughly 40 sites to monitor and work on.
Multiply that by the number of pages and social media accounts for each site and we have a serious logistics issue.