We charge our clients based on several factors, and so should you.
Clients Monthy Budget: big difference between a $1000/month client and $100,000/month same "concept" just more moving parts and larger amounts of data to analyze more often
Industry/ Purpose of the campaign: ECom, Lead Gen, Events, Awareness are charged all differently, in my opinion, knowing the value of what the success of the ads will deliver to the client will help you find an amount that can respectfully be charged for you and the client (If you care about there pockets.)
Revenue share: if it makes sense go for it, if done correctly can lead to you making more than you could have imagined and have the client happier and the value of your work will always be there.
Our skillset and historical campaign experience: If I’m killing it for other dentist lets say and consistently 6-10x ROAS then believe my service won't be "cheap" but the client will always be more then glad to pay and fork over the marketing fee (Know your worth) at the end of the day you have to send out a few dozen contracts and proposals and see what sticks always aim higher, you'll thank me later...
PM me what you dealing with and I'll guide you in the right direction.
Good luck and always be black and white with your clients the numbers don't lie, people do....