The amount of money you can earn as a contractor depends heavily on your brand. From what I've heard from contractors, their playbook seems to be the following:

1. Work to increase strength of brand to get more leads.

2. When number of leads exceeds capacity, increase prices until number of leads are manageable.

3. Go to step 1.

The hard thing here is of course to increase the strength of your brand. The easiest way to do that is to pidgeonhole yourself into a very narrow area of specialization (SEO, effectively), attend / speak at relevant conferences, write books / articles about it.