Simple answer: charge enough to make enough to have the kind of life you want to have.
Longer answer: there are a few ways to think about this – Hourly, Project Based, and Ownership.
To work out an hourly rate, you can figure out what the industry rate is for your type of work in your area, and charge somewhere around there. You’ll likely end up at around $25/hr on the low end, and $100/hr or more on the higher end.
In order to make a professional salary doing that ($75k/year or more), you’ll need to work a certain number of hours every week that turns into ~$1,500. So $50/hr x 30 hours, or $100/hr x 15 hours.
Project based is simpler than hourly, and potentially more profitable if you do it right. Figure out how much the project is worth to your client, and that’s how much you should charge. That is a longer conversation than “I charge $35/hour”, but it also is the next level up where they’re hiring you for your experience as much or more than they’re hiring you for your skills.
If you can charge $1,500 or more per project, you could do 4 projects a month and make ~$75k/year. Then it’s a matter of building a system that can generate at least one project per week, from finding the client, onboarding them, doing the work, and delivering the project.
Lastly is equity/ownership. Owning an asset that compounds or a part of a business that pays dividends can allow you to make money without trading your time for that money.
Record an album, put it up on iTunes for people to purchase, make money while you sleep.
Make enough records and sell enough copies, you’ll make even more. Find other ways to make money from that album, like licensing the music for a fee, and your ownership of that recording is worth even more.
The “right” answer of what to charge will be different for everyone, and may even include a combination of all three. But think about how you want to make your money and change accordingly.