What services are you offering for the client, and how many hours a week will you be working for the client to provide those services?(whatever you think it may be, add more onto that figure)
What is your overhead going to be? (whatever you think it may be, add more onto that figure)
How much will your E&O insurance premium be?
Just looking at those three things, let's pick some random figures. We will say you will work 20 hours a week for the client, and have $200 a month in expenses for that client. If you charge the client $1000 a month, you will net $800 after expenses. Take that $800 and divide it by 80 hours, which is what you are working for a month. That means you are getting paid $10 an hour. Less than minimum wage in many places. Are you worth more than $10 an hour? What do you think the client would pay an employee to do the same work for a month? And it would be not only their wage, but taxes for that employee, and maybe benefits too.
Only you know what your expenses will be, and how much time you will need to spend working an account for a client, but the formula pretty much stays the same. Don't sell yourself short, is the bottom line.