Since I'm also working on my own projects, I don't usually work a full time equivalent or anything like that. Pricing might be different if I had to fill all my time with work.
Let's say I can fill 10 days per month with freelancing, that's 50% occupancy. After taxes and costs that boils down to about 3k per month of income - roughly equivalent to a mid-tier job in Germany. Again, if you're just starting out, your calculation might look very different.
But if there is one piece of advice I can give: gravitate towards higher-paying clients, even if the work is not as interesting or more demanding. Higher-paying clients in general tend to be more appreciative and more competent, on top of the immediate advantages for your wallet. Only take on jobs where all parties can feel satisfied afterwards.