I have started client SEO this year and I charge 5000/m as my agency retention and tailored packages can go Upto 30k per month as the niches which I take are not local or for start ups.
They make serious money when they are ranked, and I have a lot of experience in ranking the sites in the niche I work in (which i show my clients too)
I decide campaigns according to the work that we have to do mostly and how much input I have to give.
If the work is done by an employee, I charge $30 per hour (approx) and if the work is done by me (funnels, onpage, structure and stuff) then I bill them $500-1000 per hour(approx) for my time.
Well we have 3 clients only but all are high tickets and I love working with them!
I do bespoke SEO service for other people who are referred by these people too (mostly consultation at 500-1000 dollars per phone call or strategy setup for a company's in house SEO)
But there's a catch, I don't work in niches where I haven't ranked anything of value. So that sets me apart from others who are trying to pitch my clients.
My clients get 3-4 SEO calls every day and most are too stupid to be honest. Working on pitches is important, case studies are important and getting the client to trust you is #1. He will trust you once you give him awesome ROI. Think about ROIs