Jan, I'm assuming case studies take a lot more time than SEO content. In my agency we have a different monthly retainer based on the effort and skill set needed for each service. Let's say you spend 6 hours on a case study and your hourly charges are $50. That comes to 300. So for 2 case studies a month you can charge $500 and if someone signs up for six months retainer, the price can go down to $450. I hope this helps.