Local SEO for one business with 5 locations. How much to charge?

Hello fellow BHW members. I had a question regarding local seo and have ran a few local seo optimizations and dabbled alittle on campaigns. For guys who dabble in Local SEO, how much is your retainer to run and manage the local seo for a client? I heard some charge a fixed monthly prices, and performance based where you get a percentage of the profit or something like that. What method do you use to calculate your pricing and why do you prefer it?

I personally only have a couple of clients and have kept them for a couple years now. I own the domains and gmbs though. There are a lot of variables that go into this. Are you established with case studies? Do you have any google reviews for your own business, or any trusted reviews online? Business name set up, website, etc? Do you look professional? People pay more because of these elements. Sales is psychological.
1) If you are generating form leads or phone calls figure out how much each one is worth. I got a lot of spam like most GMBS so I took that into consideration as well. I charge for everything and keep it simple. Averaging out every month. My flat rate slowly fluctuates as lead volume increases.
Which industry? Is it low volume high margin or high volume low margin? Does that lead have a generous life time value or do the customers only come back once (low retention).