How much can a freelance web designer earn per month?

You’re talking relatively large figures here…
Anyways, what you are saying is very true.

If you can sell services or a product in which you invested time in the past and don’t need to put time (or a large amount of time) into it, then you have an income which counts up.

This is the key. To me, freelancing seems more like a bridge pillar until you manage to become a full time entrepreneur, selling products and services online, making profit with no, or very little expenses compared to what you get.