Just being a freelance developer instead of working at an agency won't make you that much money. You will actually need skills that are at least close to equal to the developer working at an agency AND you will also need to be at least decent at acquiring new customers. Right now, I generally make 800 Euros - 10k Euros per project and projects of up to 3k euros can usually be finished in 40 hours of work (including communication) or less (that's actually the time I'm WORKING on the project, so excluding waiting for resources etc.). This means I am able to finish up to three projects a month if all goes well and considering smaller and larger projects.
So, let's say a good month could make me about 7.5k Euros (~ 8.5k USD). Where I live, as a full stack developer, you make around 3k Euros per month. So yes, you can earn more, but generally, you'll make right about the same amount of money or just a bit over (and a couple of times a year maybe essentially more).
I, however, am not a full-stack developer; I am a UI / UX designer & front-end developer. When I still worked for an agency, I made 2.5k Euros a month (and I got a MacBook Pro, travel allowance and a parking spot in the city) and I was right between medior and senior level at that time.