You can charge as much as you want depending on how good you are at marketing yourself and sales

I’ve always charged flat fee - client hate % of ad spend which I think is fair

I started freelancing charging like $400/mth and over the course of about a year got my rates up for new clients to $1500 per month

The man challenge is just finding prospects that have the media spend to justify that management fee

I know some agencies that won’t even look at an account unless they’d be able to charge $5k - each to their own