What's Your Overhead?
Are you working from home or are you renting office space? Think about taxes, insurance and utility bills. These are all part of your expenses and you need to choose rates that will help cover those expenses.
Study Other Copywriters
Basing your rates on what others charge isn't a good idea because there are so many variables like the person's experience, location, etc. However, you can search the Internet to see what others are charging so you'll know what's out there. Use this information as a very broad guide to get an idea of how freelancers base their rates.
Your Experience Level
If you're just starting out in the field, you're not going to be able to pull as much for a project as someone with 10 years of experience. You can set your rates competitively with other freelancers in your area and your experience level. If you're charging as much or close to someone with a large amount of experience, a client is going to tend to hire the freelancer with more experience for the money.
Where Are Your Clients?
Are you targeting agencies in smaller cities? These agencies won't be able to pay the big bucks like the larger agencies.
But you shouldn't rule smaller agencies out. If you have a limited portfolio, smaller agencies are a great way to get some samples and get your foot in the door. Many of these smaller agencies have a handful of employees so they outsource copywriting and graphic design work to freelancers. If you're strictly looking for clients in bigger cities, agencies or businesses, you're able to charge more but competition with other freelancers is fierce.
How Much Were You Making?
If you've been working in a relevant field, you can consider how much you were making to help you create your rate schedule. When you branch out on your own, if you've left with no prospects, you'll be doing a lot of grunt work to get your freelancing career off the ground. The same holds true for any freelancer.
You have to consider what you were making, what you can comfortably live with and what you're expecting to make once your freelancing business takes off.
Every freelancer dreams of making six figures. Some do. Most don't. Each person's experience and location are just a couple of the many different factors that affect every person's rates.
While there's no guide that says you should charge X amount if you have X years of experience, you can weigh all of the points above to help you create a rate schedule that you can adjust and live with throughout your freelancing career.