I'm not going to claim to understand everything on that page (spanish taxes are complicated!), but nearly all autónomos pay the minimal amount payable, which is 275 euros/month. You can contribute more if you want to get more state pension, don't I don't believe many do.
Edit - it's 29.3% of the base mínima de autónomos - which is about 920 euros a month. Now, I honestly have no idea what that actually means, but it means you pay about 270 euros regardless of what you earn (it's a stupid system that benefits the well-off).
